Recent advances in nanomaterial-based sensors for detection of trace nitroaromatic explosives

Accepted Manuscript Title: Recent Advances In Nanomaterial-based Sensors for Detection of Trace Nitroaromatic Explosives Author: Farhad Akhgari Hassan Fattahi Yones Mosaei Oskoei PII: DOI: Reference:

S0925-4005(15)30055-1 http://dx.doi.org/doi:10.1016/j.snb.2015.06.146 SNB 18723

To appear in:

Sensors and Actuators B

Received date: Revised date: Accepted date:

14-2-2015 9-6-2015 30-6-2015

Please cite this article as: http://dx.doi.org/10.1016/j.snb.2015.06.146 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Graphical Abstract (for review)

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Research Highlights

Nanomaterial-based sensors are reviewed for detection of nitro-aromatic explosives. Quantum dots, Carbon, metals, and hybrid nanomaterials-based sensors are discussed.

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Detection mechanism and analytical performance of each sensor are explained.

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Revised Manuscript

Recent Advances In Nanomaterial-based Sensors for

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Detection of Trace Nitroaromatic Explosives

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FarhadAkhgari, Hassan Fattahi*, YonesMosaeiOskoei

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North-West Institute of Science and Technology, MalekAshtar University of Technology, Urmia, Iran

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*Corresponding author e-mail: [email protected], Tel: (+98)44-32259671

Abstract

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By raising the explosive-basedterrorism during the last years and environmental concerns,

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the innovation of high efficiency sensors for detection of explosives has become one of the major

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demands in societies. Because of their unique electrical and optical properties, nanomaterials show high potential to cover all the requirements for producing selective, sensitive, simple, and

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low cast sensors for trace explosives detection. This review focuses mainly on the recent advances on nanomaterial-based sensors for detection of nitroaromatic-based explosives. Especially, the optical and electrochemical detection of nitroaromatic-based explosives through quantum dots, carbon nanotubes, graphene, metal nanoparticles and the hybridnanomaterialsare discussed.Specific characteristics of involved nanomaterials, their modification, detection mechanism, and analytical aspects are reviewed in details.

Keywords: nitroaromatic explosive, nanosensor, nanomaterial, quantum-dot, carbon nanotube,graphene, hybrid nanomaterials

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1. Introduction Sinceexplosive-based weapons are simple, easy to install, and can cause enormous damagein recent years, the reliable and accurate detection of explosives hasbecomeone of the most

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important issues of international concern[1, 2].Explosives could affect the biological systems and cause both toxic and mutagenic effects.Some of explosives can rapidly penetrate the skin

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andresult in toxicity, mutagenicity, and carcinogenicity in humans and animals[3, 4].According

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to US environmental protection agency, the maximum allowable limit of trinitrotoluene (TNT) in drinking water is about 2 ppb[5].Detection of explosives is alsonecessary for homeland security,

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environment cleaning and military issues.

Traditional methods for the detection and quantification of explosives include

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chromatographic separation techniques[6], trained canine teams[7], mass spectrometry[8],

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infrared absorption spectroscopy[9], surface-enhanced Raman spectroscopy[10, 11], X-ray

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imaging[12],thermal neutron analysis[13], electrochemical procedures[14],and ion mobility spectrometry (IMS)[15]. Although these techniques have some advantages,none of them is ideal

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due to certain features such as being time consuming, expensive, lack of portability,and requiring delicate equipment.These methods also normally confined to a laboratory environment and must be operated by a trained technician.With considering these limitationsand the importance of explosive detection,major efforts have been started for developing sensitive, fast and inexpensive detection systemsthat could be used easily foron-site identification[16]. Among these efforts, Nanotechnology has played a pivotal role in production of nanosensors with low cost, portability, specificity, and ability in rapid identification.Progresses in development of nanomaterials indicate the considerablepotential in creating nanosensors for detecting explosives

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with high sensitivity.This review focuses on therecent advancesin the use of nanomaterials for the trace detection of explosives.

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2. Nitroaromatic-based explosives An explosive is a reactive material (chemical or nuclear) containing a great amount of potential

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energy that can be initiated to undergo very rapid self-propagating decomposition and produce

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an explosion[3, 17]. Nitroaromatic- andnitroamine-based explosives are very stable and more prevalent at military.Nitroaromatic explosivesare composed of a benzene ring functionalized

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with nitro or methyl groups. Because of their electron deficient aromaticstructure, other electron –rich molecules can form stacking complexes through π-π bounding.Table (1)summarizes

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commonnitroaromatic explosives with their features that weremixed together or with fuels and

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make military explosives[3, 18,19].

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Table (1). Common nitroaromaticexplosives used in military and their features [3, 18, 19].

In many cases,vapor detection of nitroaromaticexplosivesis difficult because of their low volatility (Table 1).The low volatility of these explosives, coupled with their sticky nature,makes direct trace particle detection an attractive alternative for vapor detection. As mentioned above, the detection of explosives is vital and has its own limitation. Therefore, by consideration of these limitations and the advantages of nanosensors, trying to design nanosensors for detecting nitroaromaticexplosives is completely rational tries.

3. Nanomaterial-based sensors for nitroaromatic explosive detection 3 Page 5 of 46

Matters in nano scale show different chemical and physical properties and in certain cases an enhancement occurs in physical and chemical properties [20].This enhancement can originate from various sources such as increased surface area and confinement effects[18].

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Nanotechnology, the art of design and engineer matter in nano scale, produces nanosensors for the detection of a variety of analytes. These nanosensors are designed to overcome the many

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problems and limitations of traditional detection systems such as limitations in selectivity,

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sensitivity, size, and certainly cost.

Implementingnanomaterials with the advantages such as high surface area and improved

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surface activity in designing nanosensors will providehigher signal-to-noise ratioswith unique electrical and optical properties which can be used for highly sensitive molecular detection

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[3].The detection mechanism of nanosensors for nitroaromatic explosivescould be considered in

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two main categories including electrochemically and optically.In this review article, the

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application of nanomaterials in the development of optically and electrochemically sensors for detection of nitroaromatic explosives are studied. Characteristics of each nanomaterial,

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mechanism of detection with analytical parameters are summarized.

3.1.Electrochemically sensing of explosives by nanomaterials In the surface of the electrodes, the flow of electrons and ions caused by the electrochemical reactions could produce a detectable electronic response. The inherent redox properties of nitroaromatic explosiveswhich come from theirreducible nitro groupsmake them detectable by electrochemical methods[21, 22].Carbon-, metal- andporous-naomaterials, and also their hybridsare the most commonly used nanomaterials as electrode modifiers in electrochemically detection of nitroaromatic explosives.

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3.1.1. Carbon–based nanomaterials Nowadays, carbon-based nanomaterials, including carbon nanotubes, graphene, and carbon

nanomaterials

getting

more

have properties

attraction

for

like being

their

sensing

chemically inert,

application.Carbon-based low

cytotoxicity, high

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nanoparticlesare

biocompatibility, and unique electronic properties that raising the applications of these materials

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as sensor and energy storage materials [23-28].

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Graphene is a two-dimensional carbon nanomaterial consisting of sp2 bonded carbonsfound by Novoselov et al. in 2004[29].Graphene attracts extensive attentions in both experimental and

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theoretical studies because of its unique propertiessuch as fast conductivity, high elasticity, high mechanical strength, high surface area and rapid heterogeneous rate transfer [30]. In 2010, the

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use of graphene for the detection of trinitrotoluene (TNT) was first reported by Tang et al. [31].

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They used graphene substrate as a platformfor the electrochemical determination of

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nitroaromatic explosives. As mentioned before, TNT is a good π-electron acceptor and easily absorbs onto the π-electron-rich graphene surface. Tang and coworkers could reach a linear

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relationship between current and concentration with a detection range of 1 to 200 ppb by utilizing differential pulse voltammetry (DPV). They observed thatsignals comes from reduction of the nitro groups of TNT ongraphenefilm which was deposited onto a glassy carbon electrode(GCE) in phosphate buffered saline (PBS)[31]. Chen et al. used graphene oxide as an oxidized derivative of graphene for the detection of nitroaromatic explosives[32]. They constructed graphene-based sensing platform with electrochemical reduction of graphene oxide onto a GCE and used square-wave stripping voltammetry (SWSV) technique. Due to the strong absorption of the nitroaromatics onto the electrochemically reduced graphene layer, they could determine dinitrotoluene (DNT),

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dinitrobenzene (DNB) and trinitrobenzene (TNB). In optimized accumulation potential and times, limit of detection (LOD) for DNT in 0.5M NaCl was about 8ppb. Pumeraet al. used graphene oxide amperometric detectors in microfluidics devices and

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compared the results of detecting nitroaromaticssuch as DNT, DNB and TNT obtained at graphite microparticles,bare glassy carbon and graphene oxide-modified GCE[33]. They

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demonstrated that graphene oxide provided no advantage in terms of sensitivity or selectivity

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over that of graphite microparticles, but both of them provided significantly enhanced output signals compared to bare glassy carbonelectrode.

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Pumeragroup also presentedgraphene-based electrochemical sensors for detection of 2,4,6trinitrotoluene (TNT) with LOD about 1μgml-1 in untreated seawater[34]. They investigated the

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response of electrochemically reduction of TNT on the surface of single-, few-, and multilayer

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graphenenanoribbons and campared them with graphite microparticle-based electrodes. They

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reported that according to Fig.1, there were no meaningful differences in the responses of single, few-, and multilayer graphenenanoribbons and graphite microparticles for the electrochemical

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detection of TNT.

Fig.1.TNT in untreated seawater: (a) Cyclicvoltammograms of10 μg/mL TNT. (b) Concentration dependence of TNT on graphene-single layer, -few layers and-multi layer, and graphite electrode surfaces using DPV. Reproduced from [34]by permission of Springer-Verlag.

Further, in 2013 the Pumera group examined the detection of TNT in seawater on graphenenanoribbon-modified GC (GC-NRs), graphenenanosheet modified GC (GC-NSs) and bare GCE[35]. In this work, they utilized two different form of graphene whichhad well-defined dimensions. Voltammetry of TNT was examined initially in borate buffer and subsequently in

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seawater with the optimum response observed at GC-NRs, followed by GC-NSs, and lastly the bare GCE. The LOD were found to be 0.140 ppm for GC-NRs, 0.510 ppm for GC-NSs and 0.520 ppm for GC in seawater.

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Recently, Pumera group have investigated the application of graphene and hydrogenated graphene for the electrochemical detection of TNT in seawater[36].Graphene displayed a higher

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sensitivity than hydrogenated graphene for TNT in both borate buffer and seawater solution. The

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authors proposed that this higher sensitivity is due to the fact thatgraphene contains a larger amount of aromatic rings than hydrogenated graphene, enabling larger preconcentration.The

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LOD of TNT determined for graphene was 400 ppb and for hydrogenated graphene was 500 ppb. Carbon nanotubes (CNTs), tubular carbon-based nanostructures, that can be visualized as

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graphene rolled up into a seamless cylinder, continue to receive remarkable attention in

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electrochemistry.Wang group presented the application of CNT-modified electrodes in

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adsorptive stripping voltammetry (AdSV) for detection of TNT down to the sub-μg/l level[37]. Glassy carbon electrodes modified with multi-wall carbon nanotubes (MWCNT) and the

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stripping voltammetric performance were studied using linear sweep scans (LSV).Strong interfacial accumulation of TNT at CNT-modified electrodes that can serve as preconcentration step for AdSV lead to a detection limit of0.6 μg/l in this system. Sensitive and good linear responses for a wide range of concentration displayed the ability of this sensor for detecting ultra-trace concentration of TNT in seawater. Zhang et al. fabricated triphenylene (TP) functionalized MWCNTs material sensitive to TNT[38]. Theyinvestigated theoretically the interaction between TP and TNT by calculating their electrostatic potentials, and secondly characterized this interaction by the fluorescence spectra. In this work, Zhang et al. reported that π-conjugate structure of TP allows it to well

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functionalize onto CNT surfaces through π–π interactions. They also proposed that TP possesses a nucleophilic structure, while the TNT molecule shows electrophilic property, which leads to a significant π–π intermolecular charge transfer between them and may help the molecule

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recognition of TP to TNT.The LOD for TNT at the TP-modified electrode was shown to be 5

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ppb.

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3.1.2. Metal-based nanomaterials

Metal-basednanoparticles (such as gold, titanium, platinum and etc.)have been used for

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explosivesdetection as they can lead to enhanced sensitivities especially in electrochemical systems and in optical sensors due to increased surface areas, catalytic effects and improving

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electrical conductivity.The use of a wide variety of nanoparticles for the electrochemically

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detection of trace explosives are summarized below.

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Filanovsky et al. have described the application of modified carbon electrodes with mesoporous titanium dioxide, which acts as a support with deposited nanoparticles of ruthenium,

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platinum, or gold for detection of TNT[39]. Well separated reduction signal of TNT from oxygen reduction signals, high sensevity and good linearity between peak currents and concentration of TNTwere the advantages of TiO2–PtNP and TiO2–AuNP electrodes applied in this work. In another work, Willneret al. used gold nanoparticles for improving the sensitivity in electrochemically detection of TNT[40]. Aggregation of Au NPsbridged by oligoaniline units on the Au electrode yielded a functionalized electrode to detect parts per trillion (ppt) concentrations ofTNT. This was achieved by tailoring π-donor–acceptor interactions between TNT and π-donor-crosslinkedAuNPs. Further, the sensitivity enhanced and reach to 200 pM of

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TNT by imprinting molecular recognition sites into the π-donor oligoaniline-cross-linked Au NPs. 3.1.3.Nanoporous materials

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Efforts in seeking and preparing inorganic materials sensitive to nitroaromatic compounds have led to find out that silica- and carbon-based nanoporousmaterials showhigh sensitivity

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toward these compounds.The sensitivity of nanoporous materials towards nitroaromatics comes

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from the strong adsorption of the nitroaromatics at the large surface area of these nanomaterials. Zhang et al. has reported an electrochemical sensor for ultratrace nitroaromatic compounds

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using mesoporous SiO2 of MCM-41[41].In this work, GCE was modified with MCM41mesoporous material, and alsowithSiO2nanospheres for cathodicvoltammetric detection of

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TNT, TNB, DNT and DNB.The results showed that GCE modified with MCM-41 mesoporous

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has high sensitivity for nitroaromatic compoundsin comparison with SiO2nanosphere-modified

0.6 and 0.4 ppb, respectively.

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GCE due to its higher surface area.ReportedLODs for TNT, TNB, DNT and DNB were 0.4, 0.6,

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Another example for the application of mesoporous SiO2 as electrochemical sensor for detection of nitroaromatic compounds were reported by Shi et al.[42]. In this work, GCE was modified with mesoporousSiO2, and poly (diallyldimethylammonium chloride) (PDDA) was used for the detection of TNT, TNB, DNT and DNB in KCl. By applying DPV technique, detection limits for TNT, TNB, DNT, and DNB were about 0.3, 0.4, 0.4 and 0.3 ppb, respectively.

Trammell et al. utilizednanoporousorganosilicas

as preconcentration materials for the

electrochemical detection of TNT[43]. Imprinted and nonimprintedbenzene(BENZ)-bridged and diethylbenzene(DEB)-bridged polysilsesquioxanemesoporousorganosilicaswere immobilized on the surface of a GCE for detection of TNT in PBS solution. Imprinted BENZ due to faster release of TNT presents less peak tailing and enhanced sensitivity in comparison with imprinted 9 Page 11 of 46

DEB.The authors also reported that the imprinted DEB demonstrated significantly better TNT adsorption efficiency from an aqueous extract of contaminated soil. Although silica materials have advantages including large specific surface area and uniform

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pore size distribution, these materials have the intrinsic drawback of poor conductivity that limits their applications in electrochemical applications. In comparison with silica-based nanoporous,

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carbon-based nanoporous in addition to having large specific surface area and uniform pore size

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distribution, offers much superior conductivity, too. These advantages showed thatnanoporous carbon could be used for detection of ultratrace explosive materials.

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Zhang et al. reported the modification of GCEs with ordered mesoporous carbon(OMC) for detection of ultratrace TNT, DNT and DNB [44].Ultratrace levels of 0.2 ppb TNT, 1 ppb 2,4-

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DNT and 1 ppb 1,3-DNB can be detected in this work.The authors also mentioned that this high sensitivity comes from the high specific surface area of the OMC (745.9m2 g−1), which is much

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higher than that of other carbon materials such as carbon nanotubes (less than 200m2 g−1). Pumera group described the application of nanoporous carbon materials for electrochemical

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sensing in more details[45]. In this work, they examined NPC materials for electrochemical sensing of TNT and compared the data with those collected from CNT, graphite and bare GC electrodes. They reported that nanoporous carbon materials exhibit a faster heterogeneous electron transfer than graphite and pure carbon nanotubes, which resulted to a more sensitive response as shown in Fig. 2.

Fig. 2.Differential pulse voltammetry of 20 ppm of 2,4,6-trinitrotolueneat nanoporous carbon, graphite, CNT, and bare GC surface.condition: 20 mm borat buffer at (a) pH 2 and (b) pH 9 Reproduced from [45] by permission of John Wiley & Sons, Ltd., USA

3.5. Hybrid nanomaterials 10 Page 12 of 46

In addition to use of above mentionednanomaterials for nitroaromatic explosives detection, the

hybrids

of

these

nanomaterials

are

also

used

for

detection.

Pt

nanoparticlesensembledongraphene hybrid nanosheet (PNEGHNs) are reported for detection of

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TNT[46]. PNEGHNs in comparison with graphenenanosheets showed advantages such as high conductivity and attractive electrocatalytic behavior because of conductivity, small size and

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uniform distribution of Pt nanoparticles. Applying adsorptive stripping voltammetric (ASV) at

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PNEGHNs modified GCE showed a wide linear range and limit detection about 0.3 ppm for TNT.

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Ionic liquid /graphene hybrid electrode is also used for detection of TNT [47]. The ionic liquidgraphene-hybrid composite was prepared by combining ionic liquid and a 3D graphene

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material to yield a large specific surface area and pronounced mesoporosity. The ionic

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liquidgraphene–hybrid electrodeswere also compared with anionic liquid–carbon nanotube (IL–

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CNT) and an ionic liquid–graphite (IL–graphite) paste electrode for the detection of TNT. The voltammetry use of this sensor showed a sensitivity of 1.65 μAcm−2 per ppb and a low detection

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limit of 0.5 ppb.

In another work, Guo et al. reported porphyrin-functionalized graphene for sensitive electrochemical detection of DNT, TNT, DNB, and TNB [48]. The porphyrin/graphene sensor showed an ultratrace detection as low as 1 ppb of 2,4-dinitrotoluene, 0.5 ppb of 2,4,6trinitrotoluene, 1 ppb of 1,3,5-trinitrobenzene and 2 ppb of 1,3-dinitrobenzene. The specific and good adsorptive properties of porphyrin, large electroactive surface area and fast charge transfer of graphene were the reasons which were proposed for the high sensitivity of this sensor (Fig. 3).

Fig. 3.(a) structure of graphene and (b) the interaction betweenporphyrin and 2,4-DNT. Reproduced from [48] by permission from John Wiley, USA.

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The use of both carbon nanotubes and metallic nanoparticles for the detection of nitroaromatic explosives is reported. Hrapovic et al. reported both SWCNT and MWCNT composites with Pt, Au and Cu nanoparticles (NP) within a Nafion matrix. These sensors presented stable, reproducible and

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low background current with cyclic voltammetry and ASV. Control experiments showed no response for TNT reduction at the MWCNT-modified GCE and NP-modified GCE. A detection limit of 1 μg

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L−1was reported in tap water, river water and washed contaminated soil samples[49].

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As described in this section, coupling nanomaterials with electrochemical devices offers attractive opportunity for innovation of new and efficient sensors for trace detection of nitroaromatic

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explosives. Table 2 summarizes the results and analytical parameters of different electrochemical-

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based nanomaterialsensorsstudied herein for detection of nitroaromatic explosives.

Table 2. Different electrochemically nanomaterial-based sensors studied herein for detection

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of nitroaromatic explosives with their analytical parameters.

3.2. Optically sensing of explosives by nanomaterials 3.2.1.Quantom Dot-based nanomaterials Semiconductor

nanoparticles,

also

knownas

quantum

dots

(QDs)are

colloidal

nanocrystalline semiconductors possessing unique properties due to quantum confinement effects[50]. QDs are applied as luminescent probes forthe detection of a variety of analytes including many biological, organicand inorganic species [50-52]. Their unique optical properties

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such as broad excitation spectra and narrow emission spectra, high quantum yield and stabilitymade them to be one of the prominent nanoparticles for designing probes[51]. Quantum dots, unlike organic fluorophores, are composed of inorganic semiconducting materials, such as

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CdSe, CdS, or ZnS. In semiconducting material, the minimum energy required to cause an electron to jump from the valence band to the conduction band, referred to as the band gap, is

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constant and is considered as a property of the material.

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For quantum dots, the size of the semiconducting material approaches the Bohr radius and this produces two important electronic propertiesdifferent from that of the bulk material. First,

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the conduction band becomes separatedinto discrete sub-bands, resulting in emission of light after returning of electrons from the conduction band to the valence band. Secondly, the band

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gap of the material is no longer constant, and instead increases as the size of the quantum dot

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decreases.So, in contrast to traditional organic dyes, it is possible to tune the emission of QDs in

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the ultraviolet (UV) to near-infrared (NIR) spectral region by changing the size, morphology, or composition.Size dependent emission spectra of QDsare shown in Fig. 4(a). Fig.4(b)

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schematically explained the variation of band gap size by increasingthe size of QDs.

Fig.4.(a)Size dependent emission spectra of QDs ;(b) The variation of band gap size by increasing the size of QDs.

In a pioneering work, Goldman and co-workers developed QDs-based chemosensorfor specific detection of TNT in nanoscale[53]. The sensor consist ofa dye labeled anti-TNT specific antibody fragments attachedto the hybrid CdSe-ZnS core-shell QDs via metal-affinity coordination.Theantibody fragment in this work is black hole quencher-10 labeled with trinitrobenzene(TNB-BHQ10) which quenched the fluorescence of QDs throughfluorescence resonance energy transfer (FRET) mechanism.As shown in Fig.5, in the absence of TNT there 13 Page 15 of 46

was FRET between dye molecules and QDs while in the presence of TNT and displacing with dye molecules, the FRET was eliminated and a concentration dependent recovery of QD

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photoluminescence resulted.

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Fig.5. In the presence of TNT the FRET was eliminatedand the QD fluorescence increasing following TNBBHQ-10 release from the conjugate.Reproduced from [53] by permission of American Chemical Society, USA.

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Although QDs with no functional group were synthesized for explosive detection[54], the ion doped and surfacemodified QDs with specific functional groups were developed to improve

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the specificity and sensitivity.Tu and coworkers [55]demonstrated that the Mn2+-doped ZnSnanocrystals could be used as highly sensitive materials for the ultra-trace detection of TNT

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at soluble and atmospheric environments.The interaction between electron-rich amino ligands and electron-deficient aromatic rings of TNT quenched the QDs photoluminescence.This

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happens when electrons transfer from the conductive band of ZnS to the lowest unoccupied

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molecular orbital (LUMO) of TNT anions (Fig.6).

Fig.6.Schematic illustration for (A) the amine-capped ZnS-Mn2+nanocrystal sensors for TNT detection and (B) the quenching mechanism of fluorescence by the charge transfer from nanocrystals to TNT analytes[55].Reproduced from [55] by permission of American Chemical Society, USA.

For improvingthe selectivity of QD-based explosive sensors, molecular-imprintingtechnique and water-soluble macromolecular have been combine with sensitive QD[56, 57].Stringer and his coworkers[58]incorporated QDs into molecularly imprinted polymers (MIPs) that have an innate specificity for the analyte of interest and have applied for detection of nitroaromatic explosives.

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Amore sensitive and selective TNT sensor based on MIP/QDs systems is introduced byXu et al.[59].They used 3-aminopropyltriethoxy silane (APTES) as the functional monomer to form MIPsandtrinitrophenol (TNP) as a dummy template moleculecapped with CdTe QDs.As shown

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in Fig. 7,in the presence of TNT a Meisenheimer complex was formed between TNT and the primary amino groups on the surface of the QDs resulting in the quenching of the QDs through

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the electron transferring mechanism which allowed the TNT to be detectable optically.

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Fig.7.Schematic illustration for the preparation of DMIP@QDs and the sensing mechanism of TNT.Reproduced from [59] by permission of American Chemical Society, USA

On-site detecting of explosive from various surfaces is one of the main challenges in

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explosive detectors development.Zhang etal. [60] produceda dual-emissive fluorescent hybrid nanoparticle from two differently sized CdTe QDs emitting red and green fluorescences for

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visual detection of TNT particulates on various surfaces by ratiometric fluorescence. In the presence of TNT, the fluorescence of red QDs in thesilica nanoparticles stays constant, whereas

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the fluorescence of green QDs functionalized with polyamine quenched. Another example of QD-based nanosensors that seems to has potential for being used in practical application in real samples and on-site detection of explosives was produced by Carriónet al.[61]. This group has designed a creatinine-modified CdSe/ZnS quantum dots fluorescent probe for the determination of TNT in soil samples. For the analysis of soil samples, a solid-liquid extraction was performed and the presence of TNT quenched the original fluorescence of creatinine-QD according to the Stern–Volmer model.

Incorporation of QDs with other nanostructures or crystalline structuresprovides synergistic effect by combining chemical and physical properties of the structures. QDs incorporation with gold nanorods (AuNR) for turn-on fluorescent sensing of TNT in near-infrared region is 15 Page 17 of 46

reportedby Xiaet al.[62]. As schematically described in Fig. 8, amine-terminated AuNR and carboxyl-terminated QDs were usedto form a compact hybrid assembly through amine-carboxyl interaction with high efficiency of FRET. The TNT molecules can bind with amine groups of

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assembly.The limit of detection for TNT was about 0.1 nM.

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AuNR by forming Meisenheimer complexes and turning on the fluorescence with breaking the

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Fig.8.Structure of the hybrid AuNR-QDs assembly and a schematic illustration of its FRET-based operating principle. Reproduced from [62]by permission of American Chemical Society, USA

Studies on graphene have indicated that like QDs, itsbandgap and optical properties can be

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manipulated by decreasing its size down to 100 nm, which iscalledas graphene quantum dots (GQDs)[63, 64]. Pan et al. have developed a simple hydrothermal approach for the cutting of

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graphene sheets into surface-functionalized GQDs with diameters mainly distributed in the 5-13

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nm range and exhibited bright blue photoluminescence[65].In recent years, the application of

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GODs in optoelectronics and for biological probes has been expandeddue to their stable photoluminescence, excellent solubility, low toxicity and biocompatibility[66-68].Unlike semiconductor QDs, GQDs has π-electron-rich surface andπ-electron acceptors like TNTmolecules can easilyadsorbed on their surfaces and hence, there is no need for treatment and modification before using them.

Fanet al. developed a chemical method to prepare water soluble and surface unmodified GQDs from GO for ultrasensitive detection of TNT in solution by FRET quenching[69]. TNT molecules attached to the fluorescent GQDs platform by the π-π interactionand quenched the fluorescence of GQDs by FRET mechanism. As shown in Fig. 9(a), when the excited-state electron of the GQDs donor returns to the ground state, the ground state electrons of the TNT 16 Page 18 of 46

transit to the excited state. They also proposed that the common charge transfer from GQDs to TNT could quench the fluorescence of GQDs as shown in Fig. 9(b). In charge transfer mechanism, the excited-state electrons transfer to the LUMO of TNT and subsequently the

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electrons return to the ground state with a radiation less transition.The GQDs can sensitively

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detect down to 4.95×10-4 g/L TNT with the wide detection range (4.95×10-4–1.82×10-1 g/L).

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Fig.9.Quenching mechanism through (a) resonance energy transfer from the GQDs donor to TNT acceptor and (b) charge transfer from the excited GQDs to TNT.Reproduced from [69] by permission of Elsevier Science Ltd., UK.

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Recently li et al developed luminescent GQDs based sensor for analysis of nitrophenolbased explosives (TNP)[70].GQDs obtained from the pyrolysis of citric acid emit strong blue

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fluorescence and used directly without any modifications. TNP can quench the fluorescence of

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GQDs obtained from the pyrolysis of citric acid through FRET mechanism .The linear range and LOD for the analysis of TNPin water samples wasin the range of 0.1–15 μmol L-1and 0.091

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μmol L-1, respectively.

3.2.2. Metal and hybridnanomaterials Metal-based nanomaterials also offer some attractiveopportunity for designing optical sensors due to increased surface areas and a number of optical effectssuch as surface enhanced Raman spectroscopy, described later in this section. Jiang et al. developed a simple method for the colorimetric visualization of TNT at picomolar levels by using gold nanoparticles[71]. The method was based on the color change of Au NPs induced by the donor–acceptor (D–A) interaction between TNT and primary amines-in

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this case cysteamine which acts as both primary amine and stabilizer to theAu NPs.As illustrated in Fig. 10, addition of TNT into the aqueous solution leads to the aggregation of cysteaminestabilized Au NPs which resulted in a color change from red to violet blue (Fig.10). Jiang group

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observed that the addition of 114 pgl−1 of TNT changedthe solution color which could be visualized by the naked eye. They also investigated the potential practical application of the

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method by detection of 7×10-9 and 4×10-9 M TNT in a lake water sample and soil sample,

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respectively.

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Fig.10.a) D–A interaction between cysteamine and TNT. b) Assay for direct colorimetric visualization ofTNT based on the electron D–Ainteraction at the Au NP/solution interface.Reproduced from [72]by permission of John Wiley & Sons, Ltd., USA

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Surface enhanced Raman spectroscopy (SERS) which is one of the sensitive techniques for detection of molecule was also used for trace explosive detection[17]. Dasary et al. demonstrated

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cysteine-modified gold nanoparticle as SERS probe, for TNT recognition in 2 pico molar (pM)

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level in aqueous solution[73].In the presence of TNT,gold nanoparticles aggregate because of the

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formation of Meisenheimer complex between TNT and cysteine. As a result, it formed hot spots and provided a significant enhancement of the Raman signal intensity. Yang et al. used p-aminothiophenol(PATP)-functionalized silver nanoparticles coated on silver molybdate nanowires for detection of TNT by SERS technique[74]. PATP molecules adsorbed on silver nanoparticles undergo a catalytic coupling reaction to produce DMAB, which can form imprint molecule sites that constitute SERS ―hot spots‖. Anchoring of the TNT analyte at the hot spots leads to a significant enhancement of the Raman signal intensity.The limit of detection for TNT was about 10-12 M. Liu et al.demonstrated the fabrication of a self-assembled gold octahedral arraywith nanoscaleinterparticle gaps by a droplet evaporation process[75].In this work,Au octahedron 18 Page 20 of 46

stabilized by an ethylhexadecyldimethyl ammonium bromide (EHDAB) bilayer,which also promoted the adsorption of TNT on the composite NPs.The produced nanoparticle array provided an enhancement to the SERS detection of TNT, which led to the detection of a

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concentration as low as 10-9 M.

In recent years, modifying GO by luminescent tags such as quantum dots [76], fluorescent

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dyes and conjugated polymers[77], via covalent or non-covalent interactions was also reported.

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Zhang group presented the water-soluble GO functionalized by amine-modified mesoporous silica nanoparticles (MSNs) containing poly (p-phenylenevinylene)(PPV) via covalent

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bonds[78].They pointed that, since the fluorescent PPV macromolecules are encapsulated into MSNs, there is a large distance between the PPV and GO. So, theelectron transfercould be

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blocked andPPV@MSN can retainfluorescence properties in aqueous solution. In the presence of

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TNT, the interaction between TNT and the amino groups on PPV@MSN leads to the formation

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GO-PPV@MSN-TNT complexes thatshorten the distance between fluorescent PPV and TNT. So, the photoluminescence emission of GO-PPV@MSN can be strongly absorbed by the TNT

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derivatives, resulting in fluorescence quenching based on FRETmechanism. This sensor showed a linear response to the Sterne-Volmer equation toward TNT in the concentration range of 0-2.4 ×10-6 M and a detection limit about 1.3 × 10-7 M. The table 3 summarizes the results and analytical parameters of different optical-based nanomaterial sensors studied herein for detection of nitroaromatic explosives.

Table 3.Analytical parameters of optical-basednanomaterial sensors studied herein for detection of nitroaromatic explosives.

4. Conclusion and outlook

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High sensitive and selective detection of trace nitroaromatic explosives mostly accompany with some challenges because of the interference from other molecules or the impossibility of applying expensive and massive instruments in on-site detections.Detection of nitroaromatic

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explosives in vapor phase from a far distance is still one of the biggest challenges.

Nano scale materialsoften possess unique optical and electricalcharacteristics, whichcould

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enhance the sensitivity and selectivity with incorporation of some existing techniques.QDs due

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to quantum confinement effects exhibit attractive optoelectronic properties such as size tunable emission profile, narrow spectral bands and high emission quantum yields. These remarkable

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features nominate QDs for being used in designing new optical nanosensors for detection of trace nitroaromatic explosives. Carbon nanotubs and grapheneare the second groups that are

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nominated for this application due to theirfast conductivity, high elasticity, high mechanical

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strength, and high surface area.Metal-based nanoparticles are the other group of nanoparticles

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that could be used in both electrochemical and optical sensors due to their intrinsic features such as increased surface areas, catalytic effect, electrical conductivity and optical effects.

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In comparison of the nanomaterial-based sensorsdiscussed here, optically system generally possesses much lower LOD, even though shorter detection range is comparable.Sometimes there is a great need to detect the explosive from the safe distance. In this situation,the optically methods especially Raman-based methods are useful. Generally,electrochemically sensors need low cost instruments in comparison with Raman or fluorescent-based nanosensors. As new nanomaterials with specific characteristicsaredeveloping, analytical chemiststry to apply them for analytical purposes. Efforts in developing new sensors derive from one goal and that is producing fast, sensitive, selective, and low cast sensors.Nanomaterials, regarding their intrinsic properties, could accelerate reaching these goals. Appling data processing methods

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likechemometrics can improve the analytical efficiency of nanometial-based sensors and

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overcome theirdeficiency for detecting of nitroaromatic explosives.

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Table 1

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Table 2

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Table 3

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Supplementary Material

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Author Biographies

Farhad Akhgari has received his B.Sc. in Applied Chemistry from Urmia University, Iran, in 2010. Then he received his M.Sc. in Analytical Chemistry from the same university in 2012, where he worked on detection of environmental pollutants (e.g. heavy metals and pesticides) using chemometrics methods. Now he is a Ph.D. student in Analytical Chemistry at Urmia

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University, working on the nanomaterial-based chemical sensors.

Dr. Hassan Fattahi is an assistant professor at Malek Ashtar University of Technology (MUT),

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Iran. He started his studies from Urmia University, Iran, from which he was graduated with a B.Sc. Degree in Applied Chemistry in 2002. Then he obtained his M.Sc. and Ph.D. in Polymer

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Chemistry from University of Tabriz, Iran, in 2005 and 2012, respectively. He was on a sabbatical leave at NMR and Molecular Imaging laboratory at University of Mons, Belgium with Professor Robert N. Muller (fellow of the European society for magnetic resonance in medicine

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and biology) in 2009-2010. His main research field includes synthesis and characterization of well-defined polymers, synthesis and surface modification of nanomaterials for biomedical

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applications especially as MRI contrast agents and drug delivery systems.

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Dr. Yones Mosaei Oskoei is an assistant professor at Malek Ashtar University of Technology (MUT), Iran, where he received his B.Sc. in Applied chemistry in 2000. He received his M.Sc. and Ph.D. in Organic Chemistry form University of Tabriz, Iran, in 2004 and 2010, respectively. His main research field includes synthesis and characterization of different organic compounds especially organosilicons.

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