Polyaniline-Based Photothermal Paper Sensor for Sensitive and

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Polyaniline-Based Photothermal Paper Sensor for Sensitive and Selective Detection of 2,4,6-Trinitrotoluene Sheng Huang,† Qian He,† Suying Xu, and Leyu Wang* State Key Laboratory of Chemical Resource Engineering, Beijing Key Laboratory of Environmentally Harmful Chemical Analysis, Beijing University of Chemical Technology, Beijing 100029, P. R. China S Supporting Information *

ABSTRACT: We report for the first time a photothermal paper sensor for the selective and sensitive detection of 2,4,6-trinitrotoluene (TNT) down to 14 ng/ cm2. In the presence of TNT, a Meisenheimer complex was formed by means of a charge transfer process from an electron-rich group in polyaniline (PANI) to an electron-deficient nitro group in TNT, which resulted in the near-infrared absorption around 800 nm. Upon irradiation with an 808 nm diode laser, the photothermal effect of the PANI/TNT complex caused the temperature increase, and the temperature difference (ΔT) was proportional to the TNT concentration, while the temperature increase was hardly observed for other nitroaromatics including 2,4-dinitrotoluene (DNT), 2,4,6-trinitrophenol (TNP), and nitrobenzene (NB), affording high selectivity toward TNT. All the tests can be conducted both in solution and on paper. Therefore, the proposed photothermal strategy not only offers a fast and convenient protocol for selective detection of TNT but also indicates great potential in practical applications, especially for airport/railway security inspection and prevention of terrorist attacks. Xia and Zhu developed an exquisite fluorescence Turn-On sensor for recognizing TNT via indicator-displacement assay, where TNT binds amine-functionalized gold nanorods to release quantum dots, leading to recovery of the fluorescent signal.21 Despite high sensitivity being achieved, the applicability to on-site detection is still limited and highly desirable. As an alternative to the luminescence process, the photothermal effect, a process that involved transferring the absorbed light into heat instead of irradiation, has rarely been explored for sensing applications. Photothermal responsive materials have been extensively employed in photothermal therapy and photothermal imaging since the photothermal effect would result in hyperthermia in which it is exposed to irradiation.22,23 The photothermal technique holds great potential in terms of on-site sensing since it is easy to perform (normally carried out by a portable infrared camera), nondestructive and, more importantly, the temperature changes mainly result from the photothermal effects, thereby having low background signals from the local operating conditions and affording photothermal sensing with high sensitivity. Moreover, the photothermal sensing can be conducted on various substrates with different colors independent of background fluorescence, unlike the luminescence and colorimetric methods suffering from autofluorescence and background colors.

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ver-increasing concerns of national security and public safety have increased a wide interest in developing selective, sensitive, and rapid methods for detection of nitroaromatic explosives such as 2,4,6-trinitrotoluene (TNT)1,2 and 2,4,6-trinitrophenol (TNP or picric acid)3,4 due to their highly powerful energy which were frequently used by the terrorists. Many techniques have been developed for detecting nitroaromatic explosives such as Fourier-transform infrared (FT-IR), surface-enhanced Raman spectroscopy (SERS), 5 liquid or gas chromatography tandem mass spectrometry (LC-MS, GC-MS),6,7 and electrochemistry,8 which are either relying on sophisticated instruments or confined to time-consuming; therefore, these methods are difficult to be applied to on-site and real-time detection of explosives in a railway station or an airport. To this end, various luminescent chemosensors with high sensitivity as well as simplicity have been developed for the sensing of nitroaromatics.9−14 By conjugating luminescent QDs to antibody fragments, a pioneered immunoassay for the specific detection of TNT based on fluorescence resonance energy transfer (FRET) was developed.15 Among the reported luminescent methods for TNT detection, the fluorescence quenching (“Turn-Off”) properties were frequently employed in design fluorescent chemosensors by means of the interaction between an electron-rich amine (including primary amine, secondary amine and amide) and an electron-deficient nitro moiety through forming the Meisenheimer complex.16−20 “Turn-On” chemosensors are more desirable, though rarely reported, due to the significantly improved sensitivity by the enhancement of a photoluminescence signal with low background. For example, © XXXX American Chemical Society

Received: March 20, 2015 Accepted: April 28, 2015

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DOI: 10.1021/acs.analchem.5b01078 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry To date, the conducting polymer polyaniline (PANI) has been used for photovoltaic devices (solar cells)24 and photothermal agents for cancer therapy.25 It was found that the Lewis basic nature of PANI offers preferable binding sites for electron-efficient groups and varying the types of dopants would modulate its electronic and optical properties.26,27 The dopants (e.g., strong acid, Lewis acids) could protonate PANI to generate an interband gap state between valence and conduction bands, inducing the movement of electrons and formation of new band gap and decreasing the excitationenergy level.25,27 Based on this, the absorbance peak of doped polyaniline would red-shift, in some cases, to the near-infrared (NIR) region. It has been reported to employ the heat, generated from NIR absorption of PANI, for cancer-cell ablation.25 Since TNT is a Lewis acid with an electron-deficient aromatic ring due to three electron-drawing nitro groups, it is intriguing to employ TNT as a dopant to modulate the absorbance of PANI and subsequently the photothermal effects, which might potentially be used in the detection of TNT. Herein, we use electron-rich PANI for facile, rapid, sensitive, and selective detection and imaging of TNT explosives through TNT induced temperature changes of PANI where TNT works as a dopant to increase the NIR absorbance and accordingly the NIR photothermal effect of PANI (Scheme 1). Upon addition

Figure 1. UV-vis-NIR absorption spectra of PANI, TNT, and PANI/ TNT complexes in a mixture solvent of ethanol/acetonitrile (v/v = 1:1). The inset shows the photos of the PANI/TNT solution captured by a digital camera in daylight. The numbers on the tubes are the final concentration of TNT (μg/mL).

technique owns great potentials to be used for on-site, rapid, and selective TNT identification, which is of great importance for airport/railway security inspection and prevention of terrorist attacks.

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EXPERIMENTAL SECTION Reagents and Chemicals. 2,4,6-Trinitrophenol (TNP) and 2,4,6-trinitrotoluene (TNT) were supplied by the National Security Department of China and were recrystallized with ethanol before use. For safety considerations, all of the explosives must be kept away from fire, striking, and friction, and they should be handled carefully. 2,4-Dinitrotoluene (DNT) and nitrobenzene (NB) were purchased from the Aladdin Chemistry Co. Ltd. N-Methyl pyrrolidone (NMP), aniline, HCl, ammonium persulfate, acetonitrile, acetone, and ethanol were supplied by Beijing Chemical Factory. TNT, TNP, DNT, and NB were dissolved in a mixed solvent of ethanol and acetonitrile (volume ratio of 1:1) to obtain the stock solution before use, respectively. In brief, 8.0 mg of nitroaromatics was dissolved in 8.0 mL of the mixture solvent containing ethanol (4.0 mL) and acetonitrile (4.0 mL) to get the stock solution with a final concentration of 1.0 mg/mL. Characterization. The absorption spectra were conducted on a Shimadzu UV-3600 spectrophotometer with a spectral window range of 200−3600 nm. The thermal imaging and temperature evolution plots were performed on a FLIR-A600 infrared (IR) camera. Synthesis of PANI. Polyaniline was synthesized by chemical oxidation polymerization in the presence of excess hydrochloric acid (HCl).25 Briefly, aniline monomers (10 mmol) were added to a 1 M HCl aqueous solution (15 mL), and then, the polymerization was conducted by a dropwise addition of an ammonium persulfate (2.5 mmol) solution dissolved in a 1 M HCl aqueous solution (10 mL) as an oxidant at 4 °C for 6 h. The precipitate was collected by centrifuging (7000 r/min for 5 min) and redispersed in a 1 M sodium hydroxide solution. Then the deprotonated PANI was filtrated and redispersed in acetone to remove aniline oligomers. The dedoped PANI powder was obtained after centrifugation and dried in a vacuum oven for 24 h. The final PANI product was dissolved in mixture solvents of ethanol and acetonitrile (volume ratio of 1:1) to obtain the stock solution (0.1 mg/ mL) before use.

Scheme 1. Illustration of Photothermal Imaging and Detection of TNT by Using TNT Induced Photothermal Effects of the PANI/TNT Complex

of TNT, the resultant TNT doped PANI (PANI/TNT) has a new NIR absorption band around 800 nm (Figure 1), which displayed an NIR photothermal effect upon irradiation at 808 nm. The temperature of the PANI solution in the mixing solvent (ethanol/acetonitrile = 1:1 in volume) increases along with the increase of TNT concentration, which then could be used as a “Signal-On” reporter for constructing a photothermal sensor for TNT. It was also found that other analog nitroaromatics including 2,4-dinitrotoluene (DNT), 2,4,6trinitrophenol (TNP), and nitrobenzene (NB) showed little contribution to the photothermal effects of PANI, implying the good selectivity of the proposed method for TNT. Moreover, the TNT solution dipped on the PANI containing paper was illuminated under an IR camera via the NIR photothermal effect triggered by 808 nm-laser irradiation, and a portable and cost-effective photothermal paper sensor for TNT was developed successfully. This easily performed photothermal B

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Analytical Chemistry Photothermal Detection of TNT in Solution. Briefly, 50 μL of various concentrations of TNT or other nitroaromatics was mixed with 450 μL of the PANI solution. Thereafter the mixture solution was diluted to 0.5 mL before it was exposed to the NIR irradiation. The photothermal tests of the mixture solution were carried out with an IR camera to monitor the temperature change while the solution was exposed to irradiation at 808 nm with a power density of 1.5 W/cm2. Note: the room temperature and experimental solution temperature were shown on the FLIR camera, which were calibrated by a calibrated thermometer before carrying out each photothermal experiment. The laser fiber, the cuvette (or test paper), and the FLIR camera were spatially fixed through all of the experiments, and the power density of the laser illumination onto the sample is measured with an optical power meter and fixed in subsequent experiments. Photothermal Detection of TNT on Test Paper. The filter paper was immersed in the PANI solution for 1 h and then dried at room temperature to obtain the PANI test paper. To increase the PANI quantities on the test paper, the solvent can be replaced by N-methyl pyrrolidone (NMP) to increase the concentration of the PANI. After pressing the fingerprint dipped with various concentrations of the TNT solution (40 μL was used) on the PANI test paper, the photothermal responses of the test paper were carried out by monitoring the temperature changes with an IR camera when the paper was exposed to irradiation at 808 nm with a power density of 1.0 W/cm2. Quantities of TNT per unit area (QTNT, ng/cm2) were calculated by the formula (CTNT × VTNT)/Afingerprint, where the CTNT, VTNT, and Afingerprint are the TNT concentration, the TNT volume, and the area of fingerprint, respectively.

We further investigate the reason that TNP has no contribution to the photothermal effects of PANI. Indeed, picric acid (TNP), as its name suggests, is an organic acid. In aqueous media, it will ionize and produce the H+ ions to dope PANI, which further renders the PANI good photothermal ability, but here TNP has no influence on the TNT photothermal detection. We carried out experiments to investigate the reason. Into the PANI detection solution (EtOH/CH3CN = 1:1) we added TNT and TNP, respectively, and then the absorption spectra were carried out before and after the addition of water (Figure S2). In the case of TNT detection in the mixture media (EtOH/CH3CN/H2O = 1:1:1), a new absorption band at ∼780 nm was observed due to the formation of the Meisenheimer complex between the amine group and TNT, which is identical to that in the absence of water. As mentioned above, this NIR absorption results in the good photothermal effects under 808 nm irradiation. Base line was obtained from the organic solvent of EtOH/CH3CN = 1:1. A new absorption band at ∼970 nm in the red curve resulted from water absorption (Figure S2a). In the case of TNP, no absorption in the near-infrared (NIR) region was observed in the absence of water (Figure S2b). However, in the mixture solution (EtOH/CH3CN/H2O = 1:1:1), the PANI/TNP solution presents a new broad absorption band centered at 750 nm. This NIR absorption band can be attributed to the ionization of TNP and then protonation of PANI. In the current work, the TNT detection was carried out in organic solvents without water, and thus the TNP did not ionize. As a result, the TNP could not cause the protonation of PANI and consequently did not contribute to the photothermal effects. Therefore, TNP has no influence on the selective detection of TNT via this novel photothermal technique. To evaluate the photothermal potentials of PANI/TNT, PANI solutions containing different concentrations of TNT were placed under NIR laser irradiation (808 nm, 1.5 W/cm2). Despite that the PANI/TNT complex has maximum absorption at 437 and 775 nm, a 808 nm diode laser was chosen in light of reducing light scattering and potential interferences such as the photothermal effect of water. Figure 2a shows the evolution of temperature change (ΔT = T − T0, where T and T0 represent the final and initial temperature after and before irradiation, respectively) of the PANI solution in the presence of different amounts of TNT along with time. The PANI/TNT solution showed a rapid increase of temperature and eventually reached a plateau within 165 s, and the 808 nm NIR photothermal effect was enhanced step by step with the increase of TNT concentration from 0 to 200 μg/mL. The photothermal images captured at 180 s show the distinct temperature differences with different concentrations of TNT (Figure 2b). In addition, according to the results shown in Figure 2a, the temperature differences (ΔT) of the PANI/TNT after 3 min irradiation is linear (R12 = 0.9968 and R22 = 0.9966) to the TNT concentration in the range of (1) 1.0−10 μg/mL with a calibration function of ΔT1 = 0.09C (μg/mL) + 2.72 and (2) 10−200 μg/mL with a calibration function of ΔT2 = 0.01C (μg/mL) + 3.50 (Figure 2c). Herein, ΔT is the temperature difference (°C), and C is the concentration of TNT (μg/mL). To further demonstrate the selectivity of the PANI-based photothermal technique, three other nitroaromatics (DNT, TNP, and NB) were employed with the proposed photothermal method. Though the other three nitroaromatics, DNT, TNP, and NB, have similar chemical structures compared with TNT, no dramatic change on photothermal effects was

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RESULTS AND DISCUSSION PANI was synthesized by using anilinium salts protonated by hydrochloride (HCl) and ammonium persulfate as an oxidant. The chemical oxidative polymerization process was carried out at 4 °C for 10 h to get a dark-green precipitate. Then this precipitate was treated with an aqueous solution of NaOH to deprotonate it, affording pure PANI with blue color. The deprotonation process would ensure the amine and imine groups of PANI can be adequately exposed to TNT and induce the formation of the Meisenheimer complex by means of a charge transfer process from an electron-rich group on PANI to an electron-deficient nitro group on TNT.16−20 The formed PANI/TNT complex would extend the conjugation system and increase the electron-transfer efficiency, thus resulting in a redshift of absorption band. As expected, upon addition of TNT, an increase of absorption was observed (Figure 1) compared with that of PANI itself, which has two absorption bands at 268 and 540 nm, respectively. With the addition of more TNT, an obvious absorption peak around 780 nm was observed besides the new absorption band at 437 nm. Accordingly, the light blue color of the resultant solution turned to brown and became deeper with the increase of TNT concentration (Figure 1, inset photo). Additionally, there was no obvious absorption around 780 nm by adding other nitroaromatics including NB, DNT, and TNP, indicating that PANI cannot interact with them to form the Meisenheimer complex (Supporting Information Figure S1). Despite that both control compounds and TNT contain a nitro group, being with three nitro groups, TNT has its own unique properties to form the Meisenheimer complex, which has been extensively employed in construction of TNT responsive sensors.16−20 C

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Analytical Chemistry

Figure 3. Temperature differences (ΔT) of the PANI solution in the presence of different nitroaromatics (0.5 mg/mL) under irradiation at 808 nm (1.5 W/cm2) recorded by an IR camera. The corresponding photothermal images on the right side demonstrate the final temperature after irradiation for 180 s.

Figure 4. Photothermal images of printing paper written with PANI (a, c, and e) and PANI/TNT (b, d, and f) solutions as ink under 808 nm irradiation (1.0 W/cm2) for 0 (a, b), 2.5 (c, d), and 5.0 s (e, f), respectively. The concentration of the PANI solution is 0.1 mg/mL, and the concentration of TNT in the PANI/TNT solution is 1.0 mg/ mL. Final quantity of TNT on the printing paper is 15 μg/cm2 calculated by (20 μL × 1.0 (mg/mL)/1.33 cm2).

effect (Figure 4e) by prolonging the irradiation time to 5 s, which seems to be induced by the photothermal effect of PANI. When it comes to the one written by PANI/TNT ink, the trail of “TNT” came out upon irradiation with 808 nm light (1.0 W/ cm2) within 2.5 s, and more clear patterns were observed by prolonged irradiation time to 5.0 s. Meanwhile, it appears that all the area of the printing paper was lit up, which could be ascribed to the following reasons: 1) when we used the PANI/ TNT solution as ink to write the letters “TNT”, the ink would be diffused to the area nearby, which would show an enhanced photothermal effect compared with the plain printing paper; 2) owing to the heat conductivity of the printing paper, a local temperature increase would be transferred to the surrounding area, which leads to a temperature gradient in Figure 4f, with “TNT” letters having the highest temperature. As aforementioned in the case with PANI as ink, the printing paper has negligible photothermal effects; therefore, it was the photothermal effect of PANI/TNT that results in the temperature increase in Figure 4d and 4f. Moreover, compared with that written with the PANI solution, the “TNT” pattern with the PANI/TNT solution indicated a greatly contrast-enhanced photothermal image. These primary results suggest this photothermal imaging technique would be applicable for the construction of the TNT paper sensor. Inspired by the preliminary results, we further optimized the paper sensor to make it more suitable for photothermal analysis in a practical situation. The paper sensor was prepared by

Figure 2. (a) Temperature changes of PANI/TNT with different concentrations of TNT under irradiation at 808 nm with a power density of 1.5 W/cm2 recorded by an IR camera; (b) Photothermal images of PANI/TNT solutions containing different concentrations of TNT under irradiation at 808 nm for 3 min; (c) Calibration plot of temperature changes versus TNT concentration. ΔT = T − T0. Herein, T and T0 represent the final and initial temperature after and before irradiation, respectively.

observed (Figure 3), which agrees well with the absorption results, implying that the newly developed assay method is highly selective for TNT. To construct the TNT photothermal paper sensor, initially we fill a new pen with the PANI or PANI/TNT (1 mg/mL TNT) solution as ink and write “TNT” on the printing paper. Judging from the photothermal images written by PANI ink, it was clearly found that the 808 nm laser can hardly induce the photothermal effect of the printing paper itself (Figure 4a, 4c, 4e, background), which indicated that the NIR irradiation based photothermal sensing strategy has low background noise; and the area where “TNT” was written has a slightly photothermal D

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Analytical Chemistry immersing the filter paper into the PANI solution and then dried in air. A finger dipped with TNT at various concentrations was impressed on PANI containing filter paper to afford the TNT contaminated paper. Visible color changes from light blue to brown could be observed along with the increased amounts of TNT (Figure 5b, top-left photo). The

observed, and the temperature change is proportional to the concentration of TNT. Meanwhile, other nitroaromatics (DNT, TNP, and NB) with similar structures have shown negligible photothermal effects, allowing for selective detection of TNT. Additionally, a visible change in the photothermal images of the paper sensor has been observed with TNT down to 14 ng/cm2. Moreover, the PANI-based paper sensor was easily fabricated and suitable for portable and rapid on-site detection of trace amounts of TNT. The photothermal detection technique is rarely explored, though it has many advantages in terms of low background noise, high sensitivity, and great simplicity. This is the first example of a photothermal TNT paper sensor, offering great opportunities to construct various platforms for rapid and portable sensing applications.

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ASSOCIATED CONTENT

S Supporting Information *

UV−vis-NIR absorption spectra (Figure S1 and Figure S2). The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b01078.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions †

These authors contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (21475007 and 21275015), the State Key Project of Fundamental Research of China (2011CB932403), and the Fundamental Research Funds for the Central Universities (YS1406). We are also thankful for support from the “Public Hatching Platform for Recruited Talents of Beijing University of Chemical Technology”.

Figure 5. PANI containing paper sensor for photothermal detection of TNT: (a) Temperature evolution plots of the test paper with different amounts of TNT under irradiation at 808 nm (1.0 W/cm2) within 40 s. (b) The corresponding photothermal images captured by an IR camera and the visible color change captured by a digital camera (b, top-left photo).

area of a thumb finger is about 2.94 cm2, on which the TNT quantities are calculated through the aforementioned equation. Upon irradiation of 808 nm light, temperature changes (Figure 5a) and thermal images (Figure 5b) with different amounts of TNT were monitored by the IR camera. It was found that the temperature increases quickly and rapidly in the presence of TNT with a high dosage of TNT (Figure 5a, pattern (v)). As can be seen in Figure 5b, from pattern (i) to pattern (v), the photothermal images became brighter and brighter with the increase of TNT content from 0 (Figure 5b, (i)) to 14.0 μg/ cm2 (Figure 5b, (v)), with the photothermal response appreciable even at the content of 14 ng/cm2 (Figure 5b, (ii)). All the results indicate that this PANI-based photothermal imaging method is highly sensitive and selective for TNT sensing.

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REFERENCES

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CONCLUSION In summary, we developed a PANI-based photothermal paper sensor for rapid, selective, and sensitive detection of TNT. The electron-deficient nature of TNT is believed to promote the charge-transfer process from PANI to TNT, which in turn modulates the electronic and optical properties of PANI, resulting in a new absorption peak around 800 nm. Upon irradiation with 808 nm light, a temperature increase was E

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