ORIGINAL PAPER

A glassy carbon electrode modified with β-cyclodextin,multiwalled carbon nanotubes and graphene oxide for sensitivedetermination of 1,3-dinitrobenzene

Junhua Li & Haibo Feng & Yonglan Feng & Jinlong Liu &

Youcai Liu & Jianbo Jiang & Dong Qian

Received: 10 January 2014 /Accepted: 17 April 2014# Springer-Verlag Wien 2014

Abstract We are presenting a host-guest electrochemicalplatform for sensing the pollutant 1,3-dinitrobenzene. Themethod is based on the use of a glassy carbon electrode(GCE) covered with a composite made from multiwalledcarbon nanotubes and graphene oxide, and functionalizedwith β-cyclodextrin (β-CD). The resultant composite wascharacterized by scanning electron microscopy, Fourier trans-form infrared spectroscopy, thermogravimetric analysis andelectrochemical techniques. The modified GCE was used forthe sensitive detection of 1,3-dinitrobenzene (DNB) at work-ing voltages of −355 mV and −483 mV. Due to the specificrecognition property of β-CD and the excellent electronicproperties of the carbon nanomaterials, the electrode exhibitsoutstanding supramolecular recognition and enhanced elec-trochemical response to DNB compared to more conventionalelectrodes. Under optimum conditions, the peak currents varylinearly with the DNB concentrations in the range from 0.02to 30.0 μM, and the detection limit is 5.0 nM (at an S/N of 3).The electrode exhibits long-term stability and has been suc-cessfully applied to the determination of DNB in spiked soiland water samples.

Keywords Electrochemical sensor .β-cyclodextrin .

Graphene oxide . Carbon nanotubes . 1,3-dinitrobenzene

Introduction

1,3-Dinitrobenzene (DNB) is a nitroaromatic explosive wide-ly used for military munition, in civilian mining activities, butalso in terroristic activities. DNB is also an important organicraw material and has been widely adopted in the manufactureof numerous intermediates such as aniline, quinoline,azobenzene and trinitrotoluene in chemical synthesis indus-tries. These intermediate products are further used in makingexplosive materials, rubbers, pesticides, agriculturalchemicals, and solvents for coating materials and dyes. Obvi-ously, DNB can be frequently exposed to human and envi-ronment due to its wide applications. Moreover, DNB is themember of carcinogenic and persistent organic pollutantwhich shows highly negative impacts such as toxicity andpersistency within ecosystems, resulting in health problems tovariety of living beings. For instance, DNB is routinely usedto induce experimental brain stem lesions and testicular dam-age in laboratory rats [1]. Occupational exposure to DNB inhumans can cause anemia, nausea, neurological symptoms,and liver damage. Therefore, the sensitive, selective, and low-cost detection of DNB is crucial for homeland security, foodsafety and environment protections in modern society.

Nowadays, many sensors and detection methods for nitro-benzenes including DNB have been developed, ranging fromfluorescence [2] and luminescence [3] to FTIRspectroelectrochemistry [4]. Despite the significant advancesthat have been made for sensing DNB, a portable and reliablemethod for sensitive detection in a field environment stillfaces great challenges, including multiple interferences fromcommon household and personal care products. In addition,the need of relatively expensive instruments, multi-step

Electronic supplementary material The online version of this article(doi:10.1007/s00604-014-1271-0) contains supplementary material,which is available to authorized users.

J. Li :H. Feng : J. Liu :Y. Liu (*) : J. Jiang :D. QianCollege of Chemistry and Chemical Engineering, Central SouthUniversity, Changsha 410083, People’s Republic of Chinae-mail: liuyoucai@126.com

J. Li :Y. FengDepartment of Chemistry and Material Science, Hengyang NormalUniversity, Hunan, Hengyang 421008, People’s Republic of China

D. Qian (*)State Key Laboratory of Powder Metallurgy, Central SouthUniversity, Changsha 410083, People’s Republic of Chinae-mail: qiandong6@vip.sina.com

Microchim ActaDOI 10.1007/s00604-014-1271-0

pretreatment and complicated manipulation usually restrictsthe popularization of these methods for the recognition ofnitroaromatic compounds. Compared with the conventionalmethods, one of the promising approaches is based on elec-trochemical techniques. The fact that the electrochemical re-actions of different analytes take place at different potentialsprovides high selectivity. The hardware of electrochemicalsensors is rather simple and can be readily miniaturized, whichis suitable for a hand-held device. Furthermore, the particularelectrochemical activity of electron-deficient nitro-groups isbeneficial to electrochemical sensing of DNB, which has beendemonstrated in recent reports [5, 6].

Multiwalled carbon nanotubes (MWCNTs) and grapheneoxide (GO), the new forms of carbon nanomaterials, haveattracted considerable interests in analytical sciences [7, 8]since their discovery. In particular, MWCNTs are consideredto be the excellent electrode materials for preparing electro-chemical sensors or biosensors due to their large surface areas,excellent conductivities and electrocatalytic activities, whichcan greatly facilitate the electron transfer and reduce the redoxoverpotential. Moreover, GO has lots of hydrophilic groupswhich can improve the film-forming property of modifiedelectrodes. Recent studies have demonstrated that lots ofinorganic and organic chemicals such as Pd [9], TiO2 [10]and chitosan [11] can be firmly immobilized on the surfaces ofMWCNTs or GO to enhance both processibility and sensingperformances. Among these functional materials, β-cyclodextrin (β-CD) has drawn a great deal of attention dueto its outstanding inclusion function. This is because β-CD isan oligosaccharide consisting of seven glucose units whichpresent a toroidal form with a hydrophobic inner cavity and ahydrophilic outer side; these interesting characteristics canmake it bind selectively with a large variety of organic mole-cules of a hydrophobic nature into the cavities to form stablehost-guest inclusion complexes or nanostructure supramolec-ular assemblies, leading to high molecular selectivity andenantioselectivity [12–15]. And also, as β-CD is water-soluble and environmentally friendly, and can improve thedispersibility and stability of the functional materials, it is usedwidely in chemo/biosensors for bisphenol A [12], nifedipine[13], diethylstilbestrol [14] and other small molecules [15].Therefore, the combination of β-CD and MWCNTs/GOmight simultaneously possess properties of both carbonnanomaterials (large specific surface area) and β-CD (highsupramolecular recognition), which will provide potential ap-plications in the fields of sensors, electrocatalysis, electronics,etc. [16]. Accordingly, functionalizing MWCNTs/GO with β-CD could be an effective method for the extending applica-tions of nanomaterials in electroanalysis.

Herein, we describe a one-pot assembling strategy fordesigning and fabricating β-CD/MWCNTs/GO, and find theapplication of an enhanced sensing platform based on β-CD/MWCNTs/GO in the electrochemical detection of DNB.

Compared with unitary and binary modified electrodes, theβ-CD/MWCNTs/GO electrode exhibits much higher peakcurrent and lower overpotential for the DNB reduction. Thisresult reveals that the β-CD/MWCNTs/GO film not onlyshows the excellent electrical properties of carbonnanomaterials but also displays high supramolecular recogni-tion properties of β-CD. More significantly, both of the elec-trode reaction mechanism and some kinetic parameters werestudied. Finally, this method was successfully applied to de-tect DNB in real samples with a low detection limit of 5.0 nMand satisfactory recoveries between 97.0 and 104.6 %. Hence,the present work offers a new way to detect trace explosivesand broadens the applications of functional carbonnanomaterials in the environmental analysis.

Experimental

Reagents and materials

Graphite powders (spectrum pure), H2SO4, KMnO4 and H2O2

(30 wt.%) were obtained from Sinopharm Chemical ReagentCo., Ltd. (Shanghai, China, http://www.sinopharm.com/) andused for synthesizing GO. MWCNTs with 40–60 nm indiameter and 5–15 μm in length were purchased fromShenzhen Nanotech Port Co., Ltd. (Shenzhen, China, http://www.nanotubes.com.cn/). Nafion (5 wt.%) and DNB werepurchased from Aladdin (Shanghai, China, http://www.aladdin-reagent.com/). The standard stock solution of 1.0×10−3 M DNB was prepared daily by dissolving an exactamount of DNB in ethanol. Phosphate-buffered saline (PBS)of 0.1 M was employed as a supporting electrolyte, and its pHwas adjusted with 0.1 M NaOH and 0.1 M H3PO4. All otherreagents were of analytical grade and used without furtherpurification. Double distilled water was used throughout.

Apparatus

The scanning electron microscopy (SEM) images were takenby using a JSM-6700F field-emission SEM system operatingat 10 kV (JEOL, Japan, http://www.jeol.co.jp/en). The Fouriertransform infrared (FTIR) spectra were recorded on a FTIR-8,700 spectrophotometer (Shimadzu, Japan, http://www.shimadzu.com/). The thermo gravimetric analyses (TGA)were carried out with a TG209F3 thermal analyzer(NETZSCH, Germany, http://www.netzsch-thermal-analysis.com/en) at a heating rate of 10 °C min−1 in the nitrogen flowfrom 30 to 800 °C. All electrochemical measurements werecarried out on a CHI 660D electrochemical workstation(Shanghai Chenhua Co., China, http://chi.instrument.com.cn/). A conventional three-electrode electrochemical cell wasemployed with a modified glassy carbon electrode (GCE) asthe working electrode, a saturated calomel electrode (SCE) as

J. Li et al.

the reference electrode, and a platinum-wire electrode as thecounter electrode. The pHmeasurements were performed by aPHS-3C exact digital pH meter (Shanghai REX InstrumentFactory, China, http://en.lei-ci.com).

Electrode preparation

For the electrode preparation, GO was synthesized by a mod-ified Hummers’method described in our previous report [17].In brief, 1 g of graphite was firstly oxidized by 23 mL of 98%H2SO4 and 3 g of KMnO4, and then the reaction was termi-nated by the addition of redistilled water (140 mL) and 30 %H2O2 solution (10 mL). Finally, the GO product was obtainedby the centrifugation and vacuum drying. After that, 1 g ofMWCNTs was pretreated by the mixed strong acids of HNO3

and H2SO4 according to our recent publication [18], just witha different refluxing time (24 h). GCE (3 mm in diameter) waspolished to a mirror with 0.3 and 0.05 μm alumina slurries insequence on a polishing cloth, rinsed thoroughly with doubledistilled water after each polishing step, then sonicated suc-cessively in ethanol and double distilled water, and finallydried in air. For the preparation of a ternary electrochemicalsensing platform, a simple ultrasonic approach was employedto obtain β-CD/MWCNTs/GO. In a typical procedure, 25 mgof GO and 25 mg of MWCNTs were added into 50 mL ofdistilled water followed by sonication for 2 h, and then 20 mgof β-CD was added and sonicated for 1 h to form a homog-enous dispersion. Finally, 5 μL of the dispersion was coatedon the pretreated GCE with a microsyringe and dried outunder an infrared lamp, and 3 μL of Nafion was coated onthe prepared composite to avoid the surface leakage and forma consolidated film. The obtained electrode was denoted as β-CD/MWCNTs/GO/GCE. For comparison, β-CD/GCE,MWCNTs/GCE, GO/GCE and MWCNTs/GO/GCE were al-so prepared with the similar approach as described above.

Results and discussion

Characterization of materials

SEM, FTIR and TGA techniques were used to detailed char-acterization of β-CD, GO, MWCNTs and β-CD/MWCNTs/GO films. The SEM image of Fig. 1A illustrates the flake-likeshape and schistose structure of β-CD, while Fig. 1B showscorrugated and scrolled-like crumpled surfaces of GO. Thefolded and rich-wrinkled layers of GO film could enhance theelectrode active area. Fig. 1C exhibits that the unmodifiedMWCNTs twine together without any order, but after mixedwith GO and modified by β-CD, they can disperse uniformly,which is propitious for constructing the relatively homoge-neous and stable electrode (Fig. 1D). The β-CD moleculeswere covered on the surfaces of MWCNTs/GO, which could

prevent their large coalescence and aggregation. Moreover,the poriferous and crumpled surfaces of β-CD/MWCNTs/GOare beneficial to the enhancement of accumulation effect andthe increase of sensitivity of the prepared sensor. Furthermore,the functional groups of β-CD, GO, MWCNTs and β-CD/MWCNTs/GO were measured by FTIR spectroscopy (seeFig. S1) and the detailed description is given in the SupportingInformation.

The prepared β-CD/MWCNTs/GO composite and the cor-responding constituent materials were characterized by TGAto confirm the weight of β-CD in the ternary composite. Toidentify the point where the weight loss is the most apparent,the derivative weight loss curves (DTG) were also included.As shown in Fig. 2B, the weight of β-CD (curve a) declinessharply in two temperature ranges of 100–180 °C and 300–400 °C, which can be assigned to the removal of crystalliza-tion water and the dehydration of the surface hydroxy groups.The DTG result of GO (curve b) also shows two considerablepeaks of weight loss in the ranges of 100–150 °C and 200–300 °C, which are likely due to the dryness of GO and thedecomposition of labile oxygenated functional groups, respec-tively. Since MWCNTs are resistant to high temperature, theyare thermally stable (curve c) compared with GO and have alittle weight loss (2 % total weight loss), indicating the rela-tively less oxygen-containing groups grafted on the surfacesof MWCNTs. This result agrees well with that of FTIR. TheDTG curve of β-CD/MWCNTs/GO (curve d) shows threecharacteristic peaks which belong to those of the constituentcomponents, further demonstrating that β-CD was success-fully assembled on MWCNTs/GO. Furthermore, according toFig. 2A, the weight percentage of β-CD in the ternary com-posite is estimated to be about 24.5 wt.%, which is obviouslyhigher than the reported values of 13.8 wt.% [19] and21.5 wt.% [20] from other CD-CNTs composites. Such a largeamount of β-CD in the nanohybrid will increase the supra-molecular recognition and enrichment abilities of β-CD/MWCNTs/GO, resulting in the improvement of sensing per-formance of the prepared sensor.

Electrochemical characterization

[Fe (CN)6]3−/4− (1:1, molar ratio) were employed as an

electroactive probe couple to explore the electrochemical per-formances of various modified electrodes. Fig. 3A depicts theCVs of 5 mM [Fe (CN)6]

3−/4− in a 0.1 M KCl solution at GCE(a), β-CD/GCE (b), GO/GCE (c), MWCNTs/GCE (d),MWCNTs/GO/GCE (e) and β-CD/MWCNTs/GO/GCE (f),respectively. It can be seen that a pair of redox peaks appear atbare GCE. When β-CD was modified on GCE, obviousdecreases in peak current densities are observed. This factcan be ascribed to that β-CD might act as a barrier to theinterfacial electron transfer. The rich hydroxide groups on thesurface of β-CD can increase the negative charge density on

Sensitive determination of 1,3-dinitrobenzene

the electrode surface and thus hinder the mass transfer of [Fe(CN)6]

3−/4−. Moreover, the CVof GO/GCE shows a couple ofweak redox peaks. The oxygen-containing groups resultingfrom the preparation process of GO restrict the transfer ofprobe ions at the electrode, resulting in the weak electricalconductivity and much less peak currents of GO/GCE, whichis in agreement with previous report [9]. It is worth noting thatthe voltammetric response of probe ions increases significant-ly atMWCNTs/GCE, and higher redox peaks can be obtained.The enhancements in peak current densities can be attributedto the excellent electronic properties of MWCNTs. However,the redox responses of probe ions at MWCNTs/GO/GCE arebetween those at GO/GCE and MWCNTs/GCE, indicatingthat the MWCNTs/GO hybrid has been successfully modifiedon GCE. Owing to the high specific surface area, MWCNTs/GO can provide abundant binding sites for β-CD, and thisprocess will slow down the electron transfer between modi-fiers and electrode, resulting in a lower current response of theternary modified electrode than that of MWCNTs/GO/GCE.Furthermore, the active surface area of all electrodes can beobtained by CV using 5.0 mM K3Fe (CN)6 as a probe atdifferent scan rates. According to the Randles–Sevcik equa-tion ipa = (2.69×105) n3/2 D1/2 v1/2AC, where ipa is the anodicpeak current (A), n is the electron transfer number, A is theelectroactive surface area (cm2), D is the diffusion coefficientof K3 [Fe (CN) 6] in the solution (7.6×10

−6 cm2 s−1), C is the

Fig. 1 SEM images of (A)β-CD, (B) GO, (C) MWCNTsand (D) β-CD/MWCNTs/GO

Fig. 2 (A) TGA and (B) DTG curves of (a)β-CD, (b) GO, (c)MWCNTsand (d) β-CD/MWCNTs/GO

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concentration of K3 [Fe (CN)6] (mol cm−3) and v is the scanrate (V s−1), the average electroactive areas of GCE, β-CD/GCE, GO/GCE, MWCNTs/GCE, MWCNTs/GO/GCE andβ-CD/MWCNTs/GO/GCE were calculated to be 0.0604,0.0738, 0.142, 0.165, 0.178 and 0.189 cm2, respectively.

EIS was further used to determine the electron transferabilities of the modified electrodes and the correspondingtypical Nyquist plots are exhibited in Fig. 3B. The electron-transfer resistance (Rct) at the electrode is equal to the semi-circle diameter of EIS and can be used to evaluate the interfaceproperties of the electrodes. For the bare GCE (curve a), theNyquist plot presents a large semicircle with an approximatediameter of 500 Ω and an almost straight tail line. Thisdemonstrates that there exists a very low electron-transfer ratebetween the electrochemical probe [Fe (CN)6]

3−/4− and GCE.At the unitary modified GCEs, the Nyquist plots containrelatively larger semicircle diameters of about 650 Ω at β-CD/GCE (curve b) and 600 Ω at GO/GCE (curve c). Theincreased Rct can be attributed to the introduced electronega-tive groups on the GCE surface. However, the Rct values ofMWCNTs/GCE (curve d) and MWCNTs/GO/GCE (curve e)decline to about 250 and 380 Ω, respectively. It indicates thatthe conductivity of GCE could be improved by MWCNTswhich facilitate the electron transfer between the solution andelectrode interface. Finally, Rct of about 420 Ω at the ternary

hybrid electrode (curve f) further indicates the successfulmodification of β-CD/MWCNTs/GO on GCE. The imped-ance changes at the modified electrodes are consistent with theCV results, and they also confirm that the assembly processesfor sensors could be effectively manipulated.

Electrochemical behaviors of DNB

Nitroaromatic compounds in aqueous solution exhibit well-defined reductive peaks which result from the reduction ofthe nitrophenyl group to hydroxylamine, and this case canbe effectively characterized by CV. Fig. 4 shows CVs of1.0 μMDNB in 0.1 M PBS (pH = 7.0) at various electrodesin the potential range of −1.0–0.0 V with a scan rate of100 mV s−1. It can be seen that all CV curves have tworeduction signals which are likely to correspond to theformation of hydroxylamine moieties from the sequentialreduction of the two nitro groups, and the order of the reduc-tion peak currents of DNB at different electrodes is as follows:β-CD/MWCNTs/GO/GCE (curve f) > MWCNTs/GO/GCE(curve e) > MWCNTs/GCE (curve d) > GO/GCE (curve c) >β-CD/GCE (curve b) > GCE (curve a). Especially, two broadand weak reduction peaks at −0.406 and −0.566 V withcurrents of 1.833 and 2.215 μA are observed at bareGCE, while peak potentials shift positively (−0.355 V atpeakIand −0.483 V at peakII) and the currents increaseremarkably (10.176 and 11.382 μA) at β-CD/MWCNTs/GO/GCE. From the correlative reports, it can be concluded that thereduction potentials of DNB atβ-CD/MWCNTs/GO/GCE arealso higher than those at fullerenols/PANI-mixed-thiol/GCE(−0.730 V and −0.920 V) [6], nitrogen-doped graphene/GCE(−0.700 V and −0.570 V) [21], reduced graphene/GCE(−0.625 V and −0.760 V) [22], MHTPP/CNTs/GCE(−0.598 V and −0.490 V) [5] and ordered mesoporuscarbon/GCE (−0.450 V and −0.580 V) [23]. This result im-plies that the electrochemical response of DNB can be im-proved significantly by the integrated performance of thedeveloped ternary hybrid. To be specific, these are the

Fig. 3 (A) CVs and (B) Nyquist plots of (a) GCE, (b) β-CD/GCE, (c)GO/GCE, (d) MWCNTs/GCE, (e) MWCNTs/GO/GCE and (f) β-CD/MWCNTs/GO/GCE in 0.1 M KCl solution containing 5 mM K3Fe (CN)6/K4Fe (CN) 6. Scan rate of CV: 100 mVs−1; EIS: frequency range of100 kHz–0.1 Hz, operation potential of 220 mVand potential amplitudeof 5 mV

Fig. 4 CVs of 1.0 μM DNB at (a) GCE, (b) β-CD/GCE, (c) GO/GCE,(d) MWCNTs/GCE, (e) MWCNTs/GO/GCE, and (f) β-CD/MWCNTs/GO/GCE in 0.1M PBS (pH=7.0)

Sensitive determination of 1,3-dinitrobenzene

excellent electrocatalysis property of MWCNTs, preferablefilm-forming property of GO, and superior host-guest recog-nition capability of β-CD. Furthermore, the enhanced sensi-tivity of DNB molecules at β-CD/MWCNTs/GO is also dueto the strong charge-transfer interaction from the electron-richhydroxyl groups of β-CD to the electron-deficient aromaticring of DNB [6] and π-donor-acceptor interactions (or π-πstacking force) between MWCNTs/GO and DNB. As a result,the synergistic effect of β-CD/MWCNTs/GO makes it be anenhanced sensing platform to effectively detect DNB inaqueous solution. In addition, no oxidation peaks of DNBare observed at all electrodes in the selected potential range,showing an irreversible process for the electrochemicalreaction of DNB.

Optimization of analytical conditions

The optimized accumulation step in the experiments was set at0.0 V with 60 s under stirring condition. Moreover, pH valueof 7.0 and suspension of 5 μL were chosen for the electro-chemical detection of DNB. A successive 2-electron transferstep was deduced for the reduction of DNB and the detailedinformation is given in the Supporting Information.

The binding affinity between DNB guestand β-CD/MWCNTs/GO host

In order to evaluate the binding affinity of DNB guest to theβ-CD host immobilized on the MWCNTs, GO andMWCNTs/GO surfaces, the influence of increasing con-centrations of DNB on their cathodic peak currents wasinvestigated by CV in PBS. It was found that the peakcurrent reaches the maximum value while the DNB con-centration is higher than 10.0 mM. The association constant(K) of DNB with β-CD on the electrode surfaces can bededuced using the following equation derived from Langmuirisotherm [24].

DNB½ �Ipc

¼ 1

KImaxþ DNB½ �

Imax

where Ipc is the cathodic peak current for a given concentra-tion of DNB, Imax is the maximum peak current and K is thebinding constant of DNB with β-CD. The maximum currentsof peak II attain constancy of 1.345 mA at MWCNTs/GCE,1.138 mA at GO/GCE and 1.501 mA at MWCNTs/GO/GCE.The K values were calculated to be 2,430, 2,116 and2,823 M−1 from the x-intercept of the linear plot. The resultsare much higher than those of other guest molecules (such as1960 M−1 for levodopa and 1,558 M−1 for carbidopa [25])binding with the β-CD host, indicating β-CD immobilized onthe MWCNTs/GO surface has the enhanced abilities of

inclusion and recognition. From the above analysis, it can beconcluded that the electrochemical response mainly comesfrom the surface-confined DNBmolecules which are includedin the cavity of β-CD attached to the MWCNTs/GO film,while the diffusion of DNB molecules in the electrolyte solu-tion has little contribution to the current response.

Determination of DNB

Under the optimized experimental conditions, a series ofDNB solutions with different concentrations were detectedby the linear sweep voltammetry (LSV) and the results areshown in Fig. 5. It has been found that two good linearrelationships between the currents of peak II and DNBconcentrations are obtained in the ranges of 0.02–1.0 μMand 1.0–30.0 μM with linear regression equations of ipc(μA)=4.804 C (μM)+1.130 (R2=0.994) and ipc (μA)=0.443 C (μM)+5.755 (R2=0.996), respectively. Similarly,two good linear relationships between the currents of peak Iand DNB concentrations are also obtained in the ranges of0.02–1.0 μM and 2.0–30.0 μMwith linear regression equa-tions of ipc (μA)=3.927 C (μM)+0.532 (R2=0.994) and ipc(μA)=0.122 C (μM)+5.615 (R2=0.995), respectively. Fur-thermore, the detection limit was calculated to be 5.0 nMbased on signal/noise of 3. In order to make the analyticalcharacteristics of the prepared sensor clear, a comprehen-sive comparison of β-CD/MWCNTs/GO/GCE with othersensors reported for the detection of some nitroaromaticexplosives is listed in Table 1. It can be seen that, exceptporphyrin/CNTs/GCE [5], the developed sensor has a rela-tively wider linear range and a lower detection limit com-pared with other sensors. The comparison has confirmedthat the β-CD/MWCNTs/GO composite is an appropriateplatform for the electrochemical sensing of DNB. More-over, this sensor also has advantages of convenient fabri-cation, simple operation and low cost, and it may be suit-able for the on-site determination of DNB in a variety ofsituations.

Fig. 5 LSV responses at β-CD/MWCNTs/GO/GCE in PBS (pH7.0)containing different concentrations of DNB (from a to k): 0.02, 0.1, 0.3,0.7, 1.0, 2.0, 5.0, 9.5, 12.0, 20.0 and 30.0 μM

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Reproducibility, stability and interference

The reproducibility of this method has been evaluated bydetecting the LSV response of 1.0 μM DNB in PBS(pH 7.0) with the same electrode ten times and ten differentsensors prepared under the same condition. The determinationresults of the same sensor show an average peak current valueof 5.56 μAwith a relative standard deviation (RSD) of 3.8 %,while the ten electrodes demonstrate an acceptable reproduc-ibility in the current response to 1.0 μM DNB with a RSD of4.5 %. Moreover, the stability is another important parameterto estimate the sensor. In order to ensure the stability of thesensor, the peak current of the sensor after long-term storagewas measured. The sensor was stored in a refrigerator at 4 °Cwhen not used. The sensor retains 92 % of initial currentresponding to 1.0 μM DNB after storing for 10 days, and83 % of the original response is retained after 30 days. Theseresults demonstrate that β-CD/MWCNTs/GO/GCE has goodreproducibility and stability.

Interference studies were also carried out prior to the ap-plication of this sensor for the assay of DNB in real samples.The potential interfering substances were chosen from thegroup of substances commonly existed in soil and water.Tolerance limit is defined as the maximum concentration ofthe potential interfering substance that causes an error lessthan 5 % for the determination of 1.0 μM DNB. The resultsshow that 500-fold of SO4

2−, CO32−, NO3

−, ClO3−, Cl−, Br−,

NH4+, Ca2+, Al3+, 200-fold of Cu2+, Zn2+, Fe3+, Cd2+, Ni2+,

and 100-fold of bisphenol A, uric Acid, ascorbic acid, phenol,

catechol, 2,4-dichlorophenol and glucose do not affect theselectivity. The selective recognition properties of β-CD/MWCNTs/GO/GCE towards nitroaromatic compounds withsimilar structures were also evaluated. It was found that thedifferent positions of the nitro groups in 2,4,6-Trinitrotoluene(TNT), 1,3,5-trinitrobenzene (TNB), 2,4-dinitrotoluene(DNT) and 2-nitrotoluene (NT) give the reduction peaksat different potentials: −0.455, −0.596 and −0.702 V forTNT, −0.455, −0.596 and −0.702 V for TNB, −0.491and −0.689 V for DNT, and −0.723 V for NT. It has demon-strated that 10-fold of TNT, TNB, DNT and NT have noinfluence on the signals of 1.0 μM DNB.

Analytical applications

The contamination of soil and groundwater by DNB is themajor concern because soil and water are so important tohuman being and other living things. Herein, the samples of

Table 1 Comparison of major analysis performances of β-CD/MWCNTs/GO/GCE with the other sensors reported for the determination of somenitroaromatic explosives

Electrochemical sensors Technique Analyte Linear range (μM) Detection Limit(nM)

Correlationcoefficient

References

Porphyrin/CNTsa/GCEb DPVc DNBb 0.009–50 2 0.99854, 0.99569 [5]

NH3-plasma-treated MWCNTse/GCE SWVf DNTg 0.89–5.34 0.6394 0.994 [21]

Reduced graphene/GCE LSVh DNT 54.9–110 42 0.998 [22]

Ordered mesoporus carbon/GCE ASVi DNB − 1 ppb − [23]

Polyaniline Nanofibers/GCE DPV DNB 0.022–3.08 7.33 0.9983 [26]

MIPj/MWCNTs/GCE SWV DNB 0.045–8.5 25.15 0.9959, 0.9986 [27]

Mesoporous carbon material Amperometry DNB 0.0084–3.7 μg mL−1 0.0041 μg mL−1 − [28]

Porphyrin functionalized graphene/GCE ASV DNB 0–250 ppb 2 ppb − [29]

Poly[meso-tetrakis(2-thienyl)porphyrin]/GCE DPV DNT − 8 ppb − [30]

Textile-based electrode SWV DNT 0–50 μg mL− 0.38 μA mL μg−1 >0.99 [31]

Nitrogen-doped graphene/GCE LSV TNTk 0.528–8.8 130 0.996 [32]

Two-dimensional MIMl/Au nanoparticles/GCE DPV TNT 0.04–3.2 13 0.9995 [33]

Acrylic polymer (MIP) electrode DPV TNT 0.5–20 500 0.99 [34]

β-CDm/MWCNTs/GO/GCE LSV DNB 0.02–30.0 5.0 0.994, 0.996, 0.995 This work

a CNTs: carbon nanotubes; b GCE: glassy carbon electrode; c DPV: differential pulse voltammetry; d DNB: 1,3-dinitrobenzene; eMWCNTs: multiwalledcarbon nanotubes; f SWV: square wave voltammetry; g DNT: 2,4-dinitrotoluene; h LSV: linear sweep voltammetry; i ASV: Adsorptive strippingvoltammetry; jMIP: molecularly imprinted polymer; k TNT: 2,4,6-trinitrotoluene; lMIM: molecular imprinting monolayers; mβ-CD: β-cyclodextrin

Table 2 The recoveries of DNB spiked in soil and groundwater samples

Sample Spiked (μM) Found (μM) Recovery (%) RSD (%)

Soil 1 1.00 1.02 102.0 3.02

Soil 2 5.00 5.13 102.6 3.13

Soil 3 10.00 10.46 104.6 4.08

Groundwater 1 1.00 0.97 97.0 4.89

Groundwater 2 5.00 5.02 100.4 1.40

Groundwater 3 10.00 9.89 98.9 1.76

Sensitive determination of 1,3-dinitrobenzene

soil and groundwater were employed as matrixes for theanalysis applications. The collected soil samples werepretreated by the Saravanan’s method [35], while the ground-water was used without any pretreatment. Firstly, no signalsfor DNB were observed in these samples. Thus, the sampleswere spiked with DNB by adding a known amount of DNBsolution. In PBS with pH of 7.0, the samples were measuredby LSV under the same conditions for three times. The aver-age recovery and RSDwere determined and shown in Table 2.The recovery values range from 97.0 to 104.6 %, whichindicates this method is feasible for the detection of DNB inreal samples.

Conclusions

We report a ternary composite of β-CD/MWCNTs/GO whichwas facilely assembled by a one-pot ultrasonic method andthen used as sensing materials for the electrochemical detec-tion of DNB. Due to the high conductivity and excellentelectrocatalytic activity of MWCNTs/GO as well as thehost–guest interaction of β-CD with DNB, the prepared sen-sor exhibits enhanced signal for the electrochemical reductionof DNB. This platform not only shows the improved sensitiv-ity and long-term stability, but also offers the advantages ofsimplicity and efficient assay in target detection, which areessential for the DNB detection in environmental monitoring.Furthermore, this strategy has been successfully applied forthe determination of DNB in soil and water with satisfactoryrecoveries ranged from 97.0 to 104.6 %. In a word, this workprovides a general protocol to fabricate multiple sensors withternary composite as precursors for the sensitive and selectivedetection of nitroaromatic explosives.

Acknowledgments This work was financially supported by NationalNatural Science Foundation of China (No. 21,171,174), Provincial Nat-ural Science Foundation of Hunan (Nos. 09JJ3024 and 13JJ3112), Sci-entific Research Project of Education Department of Hunan Province(Nos. 12C0536 and 13K105), Provincial Environmental Science andTechnology Foundation of Hunan, the Opening Subject of State KeyLaboratory of Powder Metallurgy, and the Open-end Fund for the Valu-able and Precision Instruments of Central South University.

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Sensitive determination of 1,3-dinitrobenzene