Flexible chemiresistive sensor of polyaniline coated filter paper prepared by spraying for fast and non-contact detection of nitroaromatic explosives

Journal Pre-proof Flexible chemiresistive sensor of polyaniline coated filter paper prepared by spraying for fast and non-contact detection of nitroaromatic explosives Weiyu Zhang, Zhaofeng Wu, Jindou Hu, Yali Cao, Jixi Guo, Mengqiu Long, Haiming Duan, Dianzeng Jia

PII:

S0925-4005(19)31432-7

DOI:

https://doi.org/10.1016/j.snb.2019.127233

Reference:

SNB 127233

To appear in:

Sensors and Actuators: B. Chemical

Received Date:

27 July 2019

Revised Date:

23 September 2019

Accepted Date:

5 October 2019

Please cite this article as: Zhang W, Wu Z, Hu J, Cao Y, Guo J, Long M, Duan H, Jia D, Flexible chemiresistive sensor of polyaniline coated filter paper prepared by spraying for fast and non-contact detection of nitroaromatic explosives, Sensors and Actuators: B. Chemical (2019), doi: https://doi.org/10.1016/j.snb.2019.127233

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Article type: Research Paper Flexible chemiresistive sensor of polyaniline coated filter paper prepared by spraying for fast and non-contact detection of nitroaromatic explosives

Weiyu Zhang,a Zhaofeng Wu,a,b* Jindou Hu,b Yali Cao,b Jixi Guo,b Mengqiu Long,c Haiming Duan,a* Dianzeng Jiab*

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School of Physics Science and Technology, Xinjiang University, Urumqi, Xinjiang

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830046, China

Key Laboratory of Energy Materials Chemistry, Ministry of Education, Key

Laboratory of Advanced Functional Materials, Xinjiang University, Urumqi,

Institute of Super-microstructure and Ultrafast Process in Advanced Materials,

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Xinjiang 830046, China

School of Physics and Electronics, Central South University, Changsha 410083,

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China

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E-mail:[email protected] (Z. Wu), [email protected] (H. Duan), [email protected] (D.

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Jia)

The table of contents entry

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Flexible sensor of flower-like PANI coated filter paper was prepared by spraying for the first time, achieving the highly sensitive and fast detection of nitroaromatic explosives. Response fluctuation does not exceed 7% during the bending process and the decreases of responses are less than 7.9% after 2,000 bends of 40o due to the interlaced flower-like PANI fibers. The spraying method is suitable for economical,

efficient and large-scale preparation of paper-based flexible sensors.

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For ToC only

•The flexible sensor of flower-like PANI coated filter paper was prepared by spraying for the first time. o

The response time and recovery time to TNT, PA and DNT are no more than 8.1 and 1.9 s, and

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Highlights

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the theoretical detection limit for TNT and PA is 0.094 and 0.029 ppb,respectively.

Response fluctuation of flexible sensors does not exceed 7% in the bending process and the

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Abstract:

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decreases of responses are less than 7.9% after 2,000 bends of 40o.

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Both bending stability and high sensitivity are two important problems to be solved in developing flexible sensors. The flexible sensor based on flower-like polyaniline coated filter paper (PCFP) was developed by spraying for the first time, achieving the non-contact and fast detection of nitroaromatic explosives. Hierarchical structure, high permeability of filter paper and the interaction between amino groups of PANI and nitro groups of nitroaromatic explosives contributed to the sensitive and fast

response to 2, 4, 6-trinitrotoluene (TNT), picric acid (PA) and 2, 4-dinitrotoluene (DNT) at room temperature. The responses of PCFP to TNT, PA and DNT run up to about 237.7, 100.6 and 80.1%, respectively. The average response time and recovery time to TNT, PA and DNT are no more than 8.1 and 1.9 s, respectively. The theoretical limit of detection for TNT and PA is 0.094 and 0.029 ppb, respectively. More importantly, the special interlaced flower-like structures of polyaniline

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contributed to the good flexural stability. The response fluctuation to TNT, PA and

DNT do not exceed 7% in the bending process and the decreases of responses are less than 7.9% after 2,000 bends of 40o. In addition, the spraying method is suitable for

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economical, efficient and large-scale preparation of paper-based flexible sensors.

1. Introduction

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Keywords: flexible sensor, polyaniline, filter paper, explosives detection, spraying

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In recent years, explosive-based terrorism has grown enormously because explosive-based weapons are simple, easy to deploy, and can cause enormous

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damage [1-3]. As the war against terrorism in the Middle East draws to a close,

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escaped terrorists are speeding up their return to the rest of the world, exacerbating the risk of explosive attacks. For example, in April 2019, a terrorist bombing in Sri Lanka killed more than 200 people and injured hundreds of others, shocking the whole world. The rapid and sensitive detection of explosives, such as 2, 4, 6-Trinitrotoluene (TNT), picric acid (PA) and 2, 4dinitrotoluene (DNT), have attracted substantial efforts, especially in the field

of chemiresistive explosive sensors[1, 4-7]. Because nitroaromatic explosives generally have very low vapour pressures at room temperature (RT), both high sensitivity and fast response are necessary for the non-contact detection of explosives in actual security checks, such as airports and hotels [1,3, 4-6]. Traditional sensing materials are mostly solid powders and need rigid substrates or flexible substrates to support them [8]. It is reported that most current

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substrates including ceramics and plastics are nonbiodegradable, nonrecyclable and have poor biocompatibility [9,10]. For example, plastic requires 450 years to decompose. However, the lifetime of many sensors and flexible displays is

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on average 18 months [9,10]. As a result, electronic applications generate a

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large amount of waste. Meanwhile, the poor permeability of these current substrates hinders the effective exposure of sensing materials to the target gas,

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thus limiting the improvement of gas sensing properties [11-13].

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To overcome these drawbacks, the paper-based gas sensors have been developed and used as simple, multipurpose, flexible and disposable alternative

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device systems because of the easy biodegradation, high permeability and low cost. In this context, Liu et al. constructed the flexible NO2 gas sensors by spin

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coating the colloidal PbS quantum dots onto Al 2O3, polyethylene terephthalate (PET) and paper, respectively. The paper-based gas sensors showed the highest gas-sensing response to 50 ppm of NO2, because of the higher porosity from porous and rough nature of paper [11]. Manohar et al. reported that chemiresistive sensor based on single walled carbon nanotubes (SWCNTs) on

cellulosics (paper and cloth) could detect aggressive oxidizing vapours such as NO2 and chlorine at 250 and 500 ppb, respectively. The sensors of CNTs on 100% acid-free paper displayed a higher sensitivity than dip-coated films on plastic substrates [14]. Swager et al. mechanically drew CNTs on different type of papers to detect NH3, showing a theoretical limit of detection (LOD) of 0.36 ppm [15]. Swager et al. also reported a rapid prototyping of chemically

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functionalized CNTs on paper to produce sensor arrays, achieving the

discriminative detection of several volatile organic compounds (VOCs) [16]. Similar to CNTs, polyaniline (PANI) also was widely studied as promising

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candidates for flexible organic semiconductors in developing flexible

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electronics and sensors [17, 18]. What's more, PANI has the advantage of tunable morphology, scalable preparation, easy doping and low cost over CNTs

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[8, 19, 20]. Recently, Rutledge et al. fabricated gas sensors using electrospun

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PANI fibers doped with different levels of sulfonic acid [21]. The doped and undoped PANI fibers were excellent NH3 and NO2 sensors, respectively. Wan

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et al. successfully fabricated gas sensors using a transparent conducting film of hierarchically PANI networks on a PET substrate for the sensitive detection of

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NH3 [8]. Wang et al. adopted a paper loaded with PANI to selectively and sensitively detect TNT (in solution) down to 14 ng/cm2 using the photothermal effect of PANI [22]. However, there has been no report that the flexible chemiresistive PANI-based sensor has been used to detect explosives up to now. This may be

related to the fact that the common nitroaromatic explosives are very difficult to detect due to the low vapour pressures [1, 23, 24]. Recently, spraying deposition, a low-cost and large-scale technique to rapidly coat a polymer solution on a variety of substrates to prepare polymer films with micro-nanostructures, has been developed [25-27]. The spraying method provides a fast, convenient and low-cost scheme for fabricating flexible

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sensors. Considering the dual requirements of anti-explosive attacks and

environmental protection, we developed the flexible chemiresistive sensor

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based on polyaniline coated filter paper (PCFP) by spraying for the first time. As far as we know, this is the first report on the preparation of PCFP by

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chemiresistive PCFP sensor.

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spraying and the detection of nitroaromatic explosives using a flexible

2. Materials and methods

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2.1 Chemicals and Reagents

Aniline (99%, Alfa Aesar) was distilled before use and stored at 4 °C.

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Ammonium persulfate (98%) and HCl (38%) were purchased from Sinopharm Chemical Reagent Co., Ltd., Beijing, China. Qualitative filter papers (7 cm in

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diameter) were purchased from Fuyang North wood pulp and Paper Co., Ltd. Picric acid (PA) and 2, 4-dinitrotoluene (DNT) purchased from national standard substance center were recrystallized and dried to produce saturated vapour at RT. The methanol solution of 2, 4, 6-Trinitrotoluene (1 mg/mL, TNT) was purchased from Sinopharm Chemical Reagent Co., Ltd., (Shanghai China),

and TNT was recrystallized to produce saturated vapour at RT. Caution: The highly explosive TNT and other nitro-explosives used in the present study should be used are highly explosive and should be handled only in small quantities [5]. 2.2 Preparation of flexible PCFP First, 5 mL of aniline and 5 mL of HCl were added to 90 mL of deionized

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water and magnetically stirred for 10 min. Second, 5 g of ammonium persulfate was dissolved by stirring to 10 mL of deionized water. Third, the two solutions

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were cooled by ice water mixture to 0◦C and mixed evenly by the quick stirring. Fourth, the mixed solution was immediately sprayed onto the filter paper by

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spraying equipment for 30s and after 24 h the filter paper was soaked in

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deionized water for about 10 min to remove unreacted residues. Finally, PCFP was dried slowly at RT and cut into strips (with size of 20×5 mm 2) as sensing

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chips.

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2.3 Bending sensing tests

Fig. 1. Diagram of bending sensing tests of flexible PCFP sensor. A 500 mL of conical bottle containing solid explosive particles is sealed and heated at 40 °C for 24 h, then cooled at RT (25 ± 1 °C) for 24 h to obtain saturated explosive vapour at RT. The gas sensing performances of PCFP to the vapours of TNT, DNT and PA were tested at RT (25 ± 1 °C) by an electrochemical workstation (CIMPS-2, ZAHER ENNIUM) with a bias voltage

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of 1 V. The temperature and humidity of the testing room were controlled at 25 ± 1 °C and 35% ± 2% respectively by an air conditioning system. As shown in

Fig. 1, to facilitate sensing tests, the flexible PCFP was fixed on the ruler by the

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metal clip and adhesive tape, so that the sensor can be stably moved into

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different atmosphere (target gas and reference gas) by moving the ruler from the target gas to the reference gas. When the flexible PCFP was inserted into

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the saturated vapour of an explosive from the reference gas, the electric current

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changed. After the electric current reached a new constant value, the PCFP was then inserted into a same size conical flask full of air to recover. The response

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in electric current is defined as, Response=-ΔI/Ia=(Ig-Ia)/Ia×100%, where Ia and Ig are the electric current of the sensor in air and in explosive vapour. The

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response time is defined as the period in which the electric current of the sensor reaches 90% of the response value upon exposure to the explosive vapour, while the recovery time is defined as the period in which the electric current of the sensor changes to 10% of the response value after the explosive vapour is removed. The bending test of flexible PCFP was shown in Scheme S1. The

length of the PCFP is 2.00 cm and the width of the electrode clip is 0.25 cm. When the PCFP is fixed on the ruler with an electrode clip, the length of the exposed flexible sensor is 1.50 cm. When the bending angle is 0o, the flexible PCFP is laid flat on the ruler and the distance between the two clips is just 1.50 cm (Scheme S1b). When the flexible PCFP is bent, the exposed part of the flexible PCFP becomes an isosceles triangle, and the distance between the two

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clips is the bottom of the isosceles t0riangle. Therefore, the distances between

clips are 0.60 cm and 0.35 cm, respectively, corresponding to the bending angle

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of 40o and 80o (Scheme S1c, d). 2.4 Characterization

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Surface properties of pure filter paper and PCFP were tested by fourier

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transform infrared (FT-IR) spectrometer (Bruker VERTEX 70, Germany) using the attenuated total reflection mode. Meanwhile, the UV-vis absorbance spectra

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of samples were recorded with a UV-3900H spectrometer (Hitachi, Japan). The morphology of PCFP was observed by field emission scanning electron

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microscopy (FE-SEM, S-4800, Hitachi, Japan). In addition, the Raman spectra

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of samples were recorded by SENTERRA Compact Raman Microscope (Brooker, Germany).

3. Results and Discussion 3.1 Morphology and Surface Properties of PCFP

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Fig. 2. (a) SEM images of pure filter paper, (b-d) SEM images with different magnification of PCFP.

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It can be seen from the SEM images that the pure filter paper has a rough

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fiber structure (Fig. 2a,b), so it has good permeability due to the porous structure (Fig. S1). Macroscopically, PANI fibers are clustered and larger pores

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exist between island-like clusters (the dotted arrows in Fig. 2b). More careful observation shows that the combinations of micro/nano PANI fibers produce

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good permeability (Fig. 2c, d). One-dimensional structure of PANI and high

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permeability are very conducive to the penetration of target gas molecules and the rapid transfer of charges. In addition, one can see that PANI fibers are firmly rooted on the fibers of filter paper (Fig. S2). The flower-like interlaced PANI fibers and close connection between PANI and filter paper could contribute to the good bending stability for repeated mechanical bending.

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Fig. 3. (a) UV-vis spectrum, inset: photographs of pure filter paper and PCFP, (b) ATR-FTIR spectrum, (c) Raman spectrum of pure filter paper and PCFP.

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PANI loaded on filter paper can also be proved by the change of surface properties of filter paper before and after spraying. As shown in Fig. 3a, compared

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with pure filter paper, the UV-vis intensity of the PCFP in the ultraviolet-visible region increased significantly, especially in the visible region. The obvious contrast in UV-vis spectrum is consistent with the sharp contrast of the insets in Fig. 3a. In addition, the photograph of PCFP also reflects macroscopically the uniform growth of PANI on filter paper. As shown in Fig. 3b, compared with the pure filter paper, the

new peaks at about 1572 cm-1 and 1495 cm-1 appear in the FTIR spectrum of PCFP, which belong to C=C stretching-quinoid rings and C=C stretching-benzenoid rings of PANI, respectively [28]. Similarly, as shown in Fig. 3c, the Raman spectrum of the filter paper before and after spraying change significantly, which is consistent with the results reflected by UV-vis spectrum and ATR-FTIR spectrum. These results

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3.2 Gas Sensitive Performances of Flexible PCFP

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indicate that the preparation of PCFP by spraying is simple, economical and feasible.

Fig. 4. Flexible sensor based on PCFP (a) the electric currents of the sensor in air

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under different bending angles, Inset: image of the bent sensor, Response curves of the sensor to the saturated vapors of (b) TNT (9.1 ppb), (c) PA (0.97 ppb) and (d)

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DNT (411 ppb) at RT when tested under different bending angles. The sensing performances of flexible sensors based on PCFP at different

bending degrees were shown in Fig. 4. When the bending angle reaches 90o, it is equivalent to completely folding the flexible sensor. The electrical currents of the sensor showed negligible changes at different bending angles up to 90o (Fig.

4a). The inset shows the flexible sensors based on PCFP being bent. Fig. 4b-d shows the sensing response of the sensors to the saturated vapours of TNT, PA and DNT at RT when bent at different angles. It can be seen from Fig. 4b-d that

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the flexible PCFP exhibits good flexural stability at different bending angles.

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Fig. 5. (a) Sensing responses, (b) response times, (c) recovery times and standard

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deviation over three continuous responses of flexible PCFP, (d) sensing mechanism of TNT, PA and DNT interacted with HCl-doped PANI.

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Statistical data in Fig. 5a further show that these changes of responses to 9.1 ppb of TNT, 0.97 ppb of PA and 411 ppb of DNT do not exceed 7% during the

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bending process from 0o to 80o, displaying the good bending stability. The

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responses to TNT, PA and DNT run up to about 237.7, 100.6 and 80.1%, respectively, displaying the excellent sensitivity to nitroaromatic explosives. With the change of the bending degrees, the changes of response time and recovery time is no more than 11.8% (Fig. 5b, c). Importantly, the response time and recovery time of flexible PCFP to three nitroaromatic explosives are no more than 8.1 s and 1.9 s, respectively, showing the fast response and

recovery. This means that at RT the PCFP can detect explosives once in 10.0 s. Fast response-recovery speed, high sensitivity and excellent bending stability imply that the PCFP has potential application prospects. Although the concentration of saturated TNT vapour at RT is not the highest, the PCFP has the highest response to TNT. This may be because TNT has three nitro groups, whereas DNT has only two nitro groups, so TNT has the stronger ability to

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seize electrons from PANI (Fig. 5d). As reported, TNT is able to form

Meisenheimer complexes and acid-base pair complexes with amino groups,

even in the gas phase [29,30]. Thus, although the concentration of TNT (9.1

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ppb) is lower than that of DNT (411 ppb) [31], the PCFP still shows the

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maximum response to TNT. Similarly, PA with three nitro groups also has the strong ability to seize electrons (Fig. 5d). When PANI was exposed to PA

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vapour, the PA adsorbed on PANI rich in imino and took electrons from PANI.

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This process is equivalent to increasing the hole carrier of PANI and improves the conductivity. In addition, it is reported that PA having strong acidity can be

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used as dopant to improve the conductivity of PANI [32, 33]. The PA adsorbed on the surface of PANI may act as an acidic dopant, thus improving the

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conductivity of PANI. Although the saturated concentration of PA at RT is only 0.97 ppb [34], the above two reasons contributed to the highly sensitive response of PANI to PA.

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Fig. 6. Sensing curves of the PCFP tested before and after bending 1000 and

2000 times with a bending angle of 40° (a) 9.1 ppb of TNT, (b) 0.97 ppb of PA

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and (c) 411 ppb of DNT under different bending times, (d) sensing responses of

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TNT, PA and DNT under different bending times.

Accurate control of repeated mechanical bending with an angle of 40° of

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flexible PCFP was implemented by vernier calipers (Scheme S2). The flexural

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stability for the repeated mechanical bending was displayed in Fig. 6. When subjected to 2000 bending, the PCFP shows the decreases of 7.9, 7.9 and 7.2%

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in responses for TNT, PA and DNT, respectively. After bending 2000 times, the morphology of the PCFP has hardly changed from macroscopic to

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microcosmic (Fig. S3), compared with that in Fig. 2b, c. The PANI fibers rooted on fibers of filter paper contributed to the good flexural stability, beneficial for preserving its sensing performance to a large extent. The LOD to TNT and PA was estimated to better evaluate the sensing performances of the PCFP.

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3.3 Detection limit, reproducibility and selectivity

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Fig. 7. Relation between responses of flexible PCFP and vapor concentrations at RT of (a) TNT, (c) PA, (e) DNT; the fitting plots of response vs concentration of (b) TNT,

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(d) PA, (f) DNT.

Detection limit is an important parameter for estimating gas sensors. The

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relation between responses and vapour concentrations at RT were shown in Fig.

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7. For TNT, PA and DNT, a nearly linear dependency between the response and the concentration was observed (Fig. 7b, d, f). The linear relationship shows that the adsorption of explosives on PANI fibers has not yet reached saturation. According to the fitting results in Fig. 7b, d, f, the estimated LOD (defined as LOD = 3 SD/m, where m is the slope of the linear part of the calibration curve and SD is the standard deviation of noise in the response

curve) for TNT, PA and DNT is determined to be 0.094, 0.029 and 15.04 ppb, respectively. These results show the high sensitivity of the PCFP to TNT and

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PA.

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Fig. 8. Sensing curves of four different flexible PCFP to (a) 9.1 ppb of TNT, (b) 0.97 ppb of PA and (c) 411 ppb of DNT at 0o bending angle, (d) average sensing

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responses and standard deviation over three continuous responses.

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To determine the reproducibility of different flexible PCFPs, the sensing curves of four different flexible PCFP to 9.1 ppb of TNT, 0.97 ppb of PA and 411 ppb of

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DNT at 0o bending angle were evaluated in Fig. 8. As shown in the Fig. 8a-c, the sensing curves of four different flexible PCFPs to TNT, PA and DNT

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displayed good repeatability. Using these data, average sensing responses and standard deviation over three continuous responses of four different flexible PCFPs to TNT, PA and DNT were shown in Fig. 8d. This is shown more detail in the column statistics of the average responses of the four PCFPs (Fig. 8d) and the average responses to TNT, PA and DNT were 239.9±4.7, 101.7±2.8 and 82.3

±2.2, respectively. Good repeatability should be attributed to the uniform and firm growth of PANI fibers on filter paper. Selectivity and anti-interference capability are also important parameters for explosive sensors. Aniline, benzene, ethylenediamine, acetone and toluene are common VOCs and may interfere with the detection of explosives. These vapors are used to test the antiinterference ability of flexible PCFP. As shown in Fig. S4a-e, the responses of

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flexible PCFP to 1000 ppm of aniline, benzene, ethylenediamine, acetone and

toluene did not exceed -36%. It can be seen that the responses of flexible PCFP to these common VOCs are downward (negative) and relatively small, which

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shows that it has good anti-interference ability to common VOCs. Nitrobenzene

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and 2, 4-dinitrophenol have similar structures to TNT, PA and DNT, so they are used to evaluate the selectivity. The responses of flexible PCFP to the saturated

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vapour of nitrobenzene (300 [35] or 400 [36] ppm) and 2, 4-dinitrophenol at RT

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reached 32.6% and 24.5% (Fig. S4g, h), respectively, which were much smaller than the responses to TNT, DNT and PA, showing the good selectivity.

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Nitrobenzene and 2, 4-dinitrophenol are reported to be less capable of withdrawing electrons than TNTand DNT [29, 30]. Therefore, TNT and DNT

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have a higher ability to create charge-transfer complexes with the amino groups, induce a stronger conductivity change than other nitrosubstituted aromatic molecules of a less electron-withdrawing nature, contributing to the good selectivity. Relative humidity (RH) is a more common interference factor, which varies with the change of region and climate. Therefore, we focus on the

effect of varying RH on the flexible PCFP. As shown in Fig. S4f, the average response of flexible PCFP to 100% RH was less than -5.1%, and the response to 100% RH is downward. In addition, the impact of different RH on explosive

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detection has also been assessed.

Fig. 9. Sensing curves of flexible PCFP to (a) 9.1 ppb of TNT, (b) 0.97 ppb of PA

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and (c) 411 ppb of DNT under different humidity environments, (d) sensing

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responses, (e) response times, (f) recovery times and standard deviation over three continuous responses of flexible PCFP. The controlled humidity environments were achieved using

supersaturation aqueous solutions of different salts of MgCl2, NaCl and KNO3 in a closed glass vessel at room temperature, which yielded 33, 75, and 95% RH, respectively [37]. Solid particles of explosives were placed in the controlled

humidity environments for about 48 h to produce saturated vapors of TNT, DNT and PA at 25 ± 1 °C. At the same time, the temperature and RH of the testing room were controlled at 25 ± 1 °C and 35% ± 2% respectively by an air conditioning system. In the sensing tests, the RH of the reference air is 35%. As shown in Fig. 9 a-f, with the increase of RH, the average responses of flexible PCFP to three explosives decrease gradually, but the average response time

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increases significantly. This may be that the response of flexible PCFP to

humidity is downward, while the responses to nitroaromatic explosives are

upward. Therefore, there is competition between the two sensing signals. Under

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the condition of constant explosive concentration, the sensing signals decreases

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slightly with the increase of RH. At the same time, the average response time of the sensor increases correspondingly with the increase of RH due to the

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competition effect. This means that with the increase of RH, more time is

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needed to achieve the same response. However, since the RH of the reference air is the same during the test, the recovery time of the sensor has hardly

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changed significantly. It should be pointed out that although 33% RH and 35% RH are controlled by the supersaturation aqueous solutions of MgCl2 and an air

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conditioning respectively, the responses to TNT, DNT and PA are very close under two humidity environments, which proves that the controlled humidity produced by the two methods is feasible and reliable. This shows that RH does have an impact on explosive detection, especially on response time, but it has a relatively small impact on responses and recovery time.

Table 1. Comparison of the flexible PCFP and other chemiresistive sensors. Recovery Ref. time [6] -~4 s ~6.3 s ~7 s ~5 s

[38]

--

[40]

~75 s ~80 s ~100 s ~6 s ~5 s ~600s

[5]

[39]

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Response Response time 20% >10 min 8% 45.5% ~4.3 s 38.9% ~5.3 s 36.1% ~4.7 s 47% ~5 s 38% 10% ~30 s 2% 6.3% ~80 s 40% ~120 s 4% ~70 s 46.9% ~4 s 45.4% ~3 s 5.51% ~400s 7 237.7% ~4.6 s 80.1% ~2.8 s 100.6% ~8.1 s

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Analytes/ concentration ZnO nanowire TNT, 60 ppb SWNT TNT, 8 ppb Ni-ZnO TNT, 9.1 ppb Fe-ZnO DNT, 411 ppb Fe-ZnO PA, 0.97 ppb Titania(B) nanowires TNT, 9 ppb DNT, 180 ppb GaN/TiO2hybrids TNT, 100 ppb DNT,100 ppb SiNWs TNT, 9.1 ppb array/TiO2/rGO DNT, 411 ppb PA, 0.97 ppb Mn:ZnS NCs with TNT, 9.1 ppb 2+ 5% Mn DNT, 411 ppb MWNTs-PANI PA,1ppb WO3-dopedZnO PA, PCFP TNT, 9.1 ppb DNT, 411ppb PA, 0.97 ppb

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Sensing materials

~1.7 s ~1.9 s ~1.5 s

[4]

[2] [1]

This work

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In addition, the PCFP and the latest explosive sensors were compared and the results were listed in Table 1. As can be seen from Table 1, the PCFP

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perform better both in recovery speed and sensitivity than recent explosive sensors [4-6, 38-40]. Although the response times of the PCFP are not the

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shortest, they are less than 8.1 s, basically achieving the real-time detection of

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explosives. In particular, the PCFP is most sensitive to TNT, DNT and PA, compared with other sensors listed in table 1. The responses to saturated vapours of TNT, DNT and PA at RT are 237.7, 80.1 and 100.6%, respectively, which are about several times the responses reported in other literatures [4-6, 38-40]. It is well known that the gas sensing process occurs mainly on the surface of sensing materials [41, 42]. Therefore, the effective exposure of

sensing materials largely determines the sensor's sensing performance [43], which should be attributed to the formation process and structure of PANI prepared by spraying. After spraying, the small droplets of the PANI precursor adhered to the filter paper (Scheme S3). The precursor droplets adhered to filter paper slowly diffused on filter paper and the volume of droplets gradually decreased. The precursor then reacted to form micro/nano PANI fibers on fibers

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of filter paper.

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3.4 Analysis of Possible Sensing Mechanism

Fig. 10. Illustration of high sensitivity and good bending stability of flexible PCFP.

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In this way, the following three factors contribute to the highly sensitive and fast responses. First, flower-like structures effectively prevent from the aggregation of PANI fibers and produce a large number of gas passages (Fig. 10). As a result, the high permeability of filter paper and hierarchical flowerlike PANI allows explosive molecules sufficiently and rapidly to contact with

PANI fibers by the rapid diffusion through the gas passages (as shown by the red dot arrow in Scheme S3) [41, 44]. Second, micron-scale PANI fibers connecting the flower-like PANI like bridges, interlacing links between flowerlike PANI fibers and one-dimensional structure of PANI fibers facilitate the rapid transport and transfer of charges between PANI fibers and nitroaromatic explosives (Fig. 10, Fig. 2c, d) [6, 42, 43]. Third, the interaction between

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electron-rich amino groups of PANI and electron-deficient nitro groups of

nitroaromatic explosives also contributes to the rapid and selective responses.

Meanwhile, it can be seen that the flower-like PANI fibers rooted on the filter

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paper are interlaced with each other and still interlaced with each other in the

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process of bending, forming many stable percolation paths (Fig. 10). The interlaced flower-like structures contribute to the good flexural stability, which

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may also inspire the development of stretchable flexible sensors. It should be

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pointed out that with the increase of bending degree, the PANI fibers growing on filter paper become more discrete, which is conducive to the adsorption and

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desorption of target gas molecules. However, from the above sensing data, the average responses and response time of flexible PCFP to target analytes do not

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change significantly with the increase of bending degree. This may be due to the fact that PANI fibers growing on filter paper have formed hierarchical flower-like structures and have enough high permeability. Therefore, the enhanced permeability by the increase of bending degree hardly changes the sensing performance of

flexible PCFP. It also shows that the hierarchical flower-like structure provides a good structural basis for flexural stability of flexible sensors.

4. Conclusions In order to realize the high sensitivity and fast detection of nitroaromatic explosives and to solve the challenge brought by the traditional sensing substrates, a flexible explosive sensor based on filter paper has been prepared

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by spraying for the first time. After spraying, the PANI fibers growing on the filter paper formed a special interlaced flower-like structure, providing high

permeability and flexural stability. The responses of flexible PCFP to TNT, PA

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and DNT run up to about 237.7, 100.6 and 80.1% respectively, and the

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response time and recovery time are no more than 8.1 s and 1.9 s respectively. The response fluctuation of the paper-based sensor is less than 7% in the

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bending process and the decrease of responses is less than 7.9% after 2,000

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bends of 40o. Furthermore, the flexible sensor shows good selectivity and antiinterference ability. Importantly, our research proves that the spraying method

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is suitable for economical, efficient and large-scale preparation of paper-based

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flexible sensors.

Acknowledgements The authors thank the financial support from Natural Science Foundation of Xinjiang Uygur Autonomous Region (2019D01C019), China Postdoctoral Science Foundation (2017M613255), Joint Funds of NSFC-Xinjiang of China

(U1703251) and National Natural Science Foundation of China (21964016, 11664038, 61864011).

Notes The authors declare no competing financial interest.

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Appendix A. Supplementary data

Supplementary material associated with this article can be found in the online

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version.

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Biographies

Weiyu Zhang is a postgraduate student in School of Physics Science and Technology, Xinjiang University.

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Zhaofeng Wu received the Ph.D. Degree (2014) from the Institute of Solid State Physics, Chinese Academy of Sciences and afterwards he worked as a postdoc at Xinjiang Technical Institute of Physics and Chemistry, Chinese Academy of Sciences (2014-2016). Since June 2016, Dr. Wu has been an associate professor, master instructor in Xinjiang University. His research are mainly engaged in the nanofunctional materials and composite materials.

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Haiming Duan is a professor, at School of Physics Science and Technology, Xinjiang University and head of School of Physics Science and Technology. He received his Master Degree (1995) from Xinjiang University and Ph.D. Degree of condensed matter physics (2001) from the Institute of Solid State Physics, Chinese Academy of Sciences, respectively.

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Dianzeng Jia is a doctor, professor and doctoral supervisor in Xinjiang University. Prof. Jia is also the director of key laboratory of energy materials chemistry, ministry of education, vice president of Xinjiang University.

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Yali Cao is a doctor, professor and doctoral supervisor in Xinjiang University. Prof. Cao is mainly engaged in the research of nano-functional materials, including nanophotocatalysts, nano-electrode materials, nano-gas sensing materials, etc.

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Jixi Guo is a doctor, professor and doctoral supervisor in Xinjiang University. Prof. Guo is mainly engaged in photoelectric functional materials, etc.

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Mengqiu Long is a doctor, professor and doctoral supervisor in Central South University. Prof. Long is engaged in theoretical research on the electronic structure and charge transport properties of nanostructures and materials.