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Determination of explosives based on novel type of sensor using porphyrin functionalized carbon nanotubes
Colloids and Surfaces B: Biointerfaces 88 (2011) 396–401
Contents lists available at ScienceDirect
Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
Determination of explosives based on novel type of sensor using porphyrin functionalized carbon nanotubes Xiaoquan Lu ∗ , Yanli Quan, Zhonghua Xue, Bowan Wu, Hetong Qi, Dong Liu Key Laboratory of Bioelectrochemistry and Environmental Analysis of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China
a r t i c l e
i n f o
Article history: Received 27 February 2011 Received in revised form 28 May 2011 Accepted 6 July 2011 Available online 18 July 2011 Keywords: Porphyrin Carbon nanotubes Synergetic effect Explosive Interaction
a b s t r a c t The hydroxide of meso-tetraphenylporphyrin derivatives functionalized carbon nanotubes (CNTs) was fabricated in our research to explore the interaction between porphyrin and explosive. It was turned out that in the formation of grid porphyrin film, carbon nanotubes as a cruciul base materials promoted the electron transfer rate. Most of important, the results also showed that the electrochemical response was enhanced through increasing the number of –OH substitution in porphyrin. Such information provides the platform for a practical strategy for rational design of the sensor of explosives. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nitroaromatic compounds, which are commonly used in the manufacture of explosives, pesticides, dyes and pharmaceuticals, have drawn considerable attention to environmental monitoring. Due to the high toxicity, wide distribution, and stable chemical character [1], the rapid and precise detection of these toxic agents in the environment become increasingly important for homeland security and health protection. To our knowledge, various strategies, including fluorescence [2], spectrometer [3], microfabricated capillary electrophoresis chip [4], etc., have been conducted on explosives. Electrochemical methods seem to be excellent candidates for the rapid detection of explosives owing to their simple preparation and easier to fabricate portable detectors [5]. Recently, kinds of electrochemical sensors for detecting explosives have been explored. It was reported that attempts have been made in search of superior -electron donors to interact with nitroaromatic compounds by strong – interactions [6], or utilize novel materials [7]. Carbon nanotubes (CNTs) have been examined for the removal of contaminants particularly those having aromatic -systems from water and gases attributed to their large external surface area [8]. Recently, various studies using CNTs as adsorbents have been carried out by some groups [7,9]. However, just rely on the preconcentration capability of CNTs to detect explosives is far
∗ Corresponding author. Tel.: +86 931 7971276; fax: +86 931 7971323. E-mail address: [email protected] (X. Lu). 0927-7765/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2011.07.020
from enough. Therefore, it is necessary to employ more-compact and low-cost method to monitor explosives. Porphyrin compounds are widely employed as platform to recognize various species because of their structures with high delocalized -bonds and the unique properties of the tetraphenylporphyrin core which has ability to participate in hydrogen bonds with the anion to form the hydrogen bond donor–accepor complexes [10]. Their electrochemical and photophysical properties make porphyrin derivatives combine with a complementary electron-accepting species to mimic the electron transport of biological process based on the formation of electron donor/acceptor (D/A). W. Kutner has reported that porphyrin was able to interact with fulleren to form the supramolecular donor–acceptor hybrid, in which the acceptor was coordinated to the surface-accessible porphyrin sites [11]. Besides, due to the strong electron-withdrawing ability of the nitro group, explosives have the ability to accept electron from electron donor to form electron donor–acceptor (EDA) complexes, which involves electron transferred from the highest occupied molecular orbital (HOMO) of donor to the lowest unoccupied molecular orbital (LUMO) of acceptor unpaired electrons [12,13]. Considering porphyrin as a typical -donor compound, we envisioned the introduction of the hydroxyphenyl-porphyrin into CNTs could enhance the sensitivity of the detection of explosives based on the synergetic effects of porphyrin and CNTs (Scheme 1). It is supposed that the mechanism involved in this strategy as follows: on one hand, CNTs utilize the endemic preconcentration capability to accumulate explosives, on the other hand, the electron-deficient
X. Lu et al. / Colloids and Surfaces B: Biointerfaces 88 (2011) 396–401
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Scheme 1. Schematic representation of electrochemical sensing explosives. (A) The CNTs to glass carbon electrode surface; (B) the electropolymerization porphyrin to CNTs surface; (C) explosive adsorbs to the functionalized film; (D) electrochemical stripping detection of explosive.
nitro group of explosives interact with the electron-rich hydroxyl group of porphyrin to form acid-base paring interaction instantaneously and then the aromatic ring of explosive turn into planar and interact with the -macrocycle of porphyrin by a parallelplanar to form strong donor–acceptor (EDA) complexes, which involves electron transfer from -donor of porphyrin to electrondeficient explosive. In order to confirm the validity of this approach and the rationality of this hypothesis, we initiated a study aimed at providing a strategy for rational design of electrochemical sensor based on the 5-(4-hydroxyphenyl)-10,15,20-triphenylporphyrin (MHTPP) functionalized CNTs film. To provide further insight into the interaction between explosives and porphyrin, the CNTs was also fuctionalized by meso-tetrakis(4-hydroxyphenyl)porphyrin (THTPP) to probe the electrochemical response of explosives. 2. Experiment 2.1. Apparatus and reagents Stock solutions of m-DNB (1000 mg/L) (purchased from Beijing Chemical Co. Ltd.) were dissolved by acetonitrile (ACN). Analyte solutions were prepared by dilution the stock solutions using PBS contained 0.1 M NaCl and protected from light. Electrochemical studies were performed with CHI660 electrochemical work station (CHI, USA) using conventional three-electrode cells with bare and modified GC electrode as work electrodes, a platinum wire counter electrode, and Ag/AgCl reference electrode. The surface morphologies of the prepared electrodes were observed through scanning electron microscopy (SEM) on JSM-6701F (Japan Electron Optics Company). UV1102 spectrometer (Techcomp Bio-Equipment LTD, Shanghai, China) was used to obtain UV–vis spectra. 2.2. Preparation of MHTPP/CNTs/GCE The commercially available CNTs was treated to induce carboxylic acid groups on the surface [14]. The treated CNTs was dispersed in 2 mL N, N-dimethylformamide (DMF) with the aid of ultrasonic agitation to give 1 mg/mL black suspension. Glassy carbon electrode (GCE, 2 mm in diameter) was carefully polished and rinsed completely by distilled water. Afterwards, 5 L suspension was cast onto the surface of GCE pretreated and then dried under infrared lamp. Finally, CNTs/GCE was cleaned by ethanol and distilled water respectively. The porphyrin film was accomplished by the application of 8 potential cycles from acetonitrile and 0.1 mM porphyrin containing 0.02 M tetrabutylammonium perchlorate (TBAP) solutions as electrolyte solution at a scan rate of
50 mV/s ranging from 0.3 V to 1.2 V versus Ag/AgCl [15,16]. After covalent functionalized, the electrode was cleaned by acetonitrile and distilled water respectively to exclude residual monomer from the electrode. 3. Results and discussion 3.1. The formation of porphyrin film The available redox chemistry for hydroxyphenylporphyrins shows their dual functionality: porphyrin and polyphenol. Unprotonated nitrogen in the central region of porphyrin ring is negative charge which can lose one electron to form pyrrole radical cation, and react with carboxyl of electrode surface [15]. It is believed that the hydroxyl of benzene ring carried some unpaired electron could cause the p- conjugation between hydroxy and conjugated benzene ring, which makes the phenyl carbon atom at the meta position become radical cation and thus the nucleophilic attack of adjacent porphyrin hydroxy group takes place at its phenyl carbon atom at the meta position with respect to the OH group [17,16]. Such nucleophilic attack resulted in an ether linkage between the adjacent porphyrins at the meta carbon of the phenyl ring [18]. During the oxidative polymerization process formed a dimer-like radical-ion species and thus the peak appeared at 0.9 V in the present of porphyrin was attributed to the formation of radical-ion species. As can be seen from Fig. 1, compared with cyclic voltammograms in acetonitrile and electrolyte solution in the absence of porphyrin (Fig. 1a), the current at 0.9 V decreases upon continued electropolymerization in the present of porphyrin, indicating passivated polymeric film formed on the electrode blocking the access of the monomer to the electrode surface (Fig. 1b). However, the growth behavior on CNTs substrate (Fig. 1c) was different from that on the flat GC substrate. At about 1.08 V emerges additional peak may ascribed to self-aggregate forming poly-phenoxy selfaggregated dimer films. Furthermore, the current response at 0.9 V shifts to slight negative during electropolymerization, indicating CNTs result in the formation of porphyrin film more easily. 3.2. Characterization of different modified electrodes The scanning electron micrographs of purified CNTs and MHTPP/CNTs are showed in Fig. 2 to confirm the formation and growth of polymer film on the CNTs. The nanocomposite displayed porous structures composed of cemented entangled network structure (Fig. 2A), which is different from the structure of the purified CNTs (Fig. 2B), indicating thin layer porphyrin film
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Fig. 1. (a) Cyclic voltammograms of bare electrode in acetonitrile containing 0.02 M TBAP in the absence of porphyrin in a sequence of 8 cyclic scans and at a scan rate of 50 mV/s, (b) Cyclic voltammograms of glass carbon electrode in acetonitrile containing 0.1 mM porphyrin and 0.02 M TBAP. (c) Cyclic voltammograms of CNTs fuctionalized electrode during electropolymerization process in acetonitrile containing 0.1 mM porphyrin and 0.02 M TBAP.
electropolymerized onto CNTs. The UV–vis absorption spectrum of MHTPP, MHTPP/CNTs, and CNTs are presented in Fig. 2C. The Soret band of MHTPP/CNTs (curve b, Fig. 2C) splits into two obvious absorption band in comparison with that of the solution profile (curve a, Fig. 2C). The spectral changes result from the fact that the deprotonation of the phenolic protons increased the energy
of orbitals localized on the phenoxide anion substituents [19] and caused a charge-transfer transition from phenoxide anion to porphyrin further to the surface of CNTs and thus resulted in the aggregation of porphyrin [13,20]. Cyclic voltammetry (CV) was performed to anticipate the surface properties of different films using the redox couple Fe(CN)6 3−/4− as redox probe to identify the electron transfer efficiency. The electrochemical response was suppressed to the lowest value as MHTPP functionalized glassy carbon electrode (curve a, Fig. 2D) compared with bare glassy carbon electrode (curve b, Fig. 2D). This result demonstrates that the electron transfer was inhibited by dense nonconductive porphyrin film in which the porphyrin rings may be dense packed with a parallel orientation to the bare carbon glass electrode. Nevertheless, CNTs with large surface area as matrix (curve c, Fig. 2D), the electron transfer efficiency is promoted and analyte cross ultrathin membrane of porphyrin became more easily compared with MHTPP/GCE. Nonetheless, it should be noted that the electrochemical response was lower than CNTs/GCE (curve d, Fig. 2D) indicating the presence of porphyrin film retard electron transfer to the redox probe at the CNTs/solution interface. These results also indicate that porous CNTs film with large surface area provides ideal matrix for the distribution of porphyrin and promotes the electron-transfer reactions. So we have speculated that in the formation grid porphyrin film, carbon nanotubes acted as the crucial base materials to promote the electronic transfer rate. 3.3. Electrocatalytic detection of m-DNB and the optimization of analytical condition To verify our hypothesis of the beginning, we investigate the electrochemical responses of different modified films to the 1,
Fig. 2. (A) Scanning electron microscopy (SEM) image of MHTPP/CNTs/GCE (B) SEM image of CNTs/GCE (C) Soret band region of (a) MHTPP, (b) MHTPP/CNTs, (c) CNTs in 8:2 ethanol/acetonitrile (D) CVs of (a) MHTPP/GCE, (b) GCE, (c) MHTPP/CNTs/GCE, (d) CNTs/GCE in 5 mM Fe(CN)6 3−/4− containing 0.1 M KCl.
X. Lu et al. / Colloids and Surfaces B: Biointerfaces 88 (2011) 396–401
Fig. 3. CVs (a) GCE, (b) MHTPP/GCE, (c) CNTs/GCE, (d) MHTPP/CNTs/GCE of pH 10 contained 0.5 mM m-DNB at scan rate 50 mV/s.
3-dinitrobenzene. Fig. 3 reveals the electrochemical response of different modified films to the m-DNB. The cathodic peaks at −0.598 V and −0.714 V correspond to the stepwise reduction of nitro groups to the amine groups through the nitroso and hydroxylamine intermediates, according to the mechanism for the reduction of nitro groups [7]. Compared with the weak electrochemical response of bare electrode (Fig. 3a), CNTs/GCE (Fig. 3c) showed better reduction peaks associated with endemic preconcentration capability of CNTs to concentrate hydrophobic organic compounds at the surface of CNTs [21]. Fig. 3b indicated that compact insulated porphyrin film prevented the explosives from reaching the surface of electrode and reduced the possibility of interaction. In contrast, the response of peak current increased when CNTs acted as matrix to functionalized porphyrin (Fig. 3d), which reveals that CNTs could increase the surface area and create interstice spaces which serve as m-DNB conducting channels. Meanwhile, the attachment of porphyrin ultrathin film on CNTs provides more receptor sites to explosive compound and leads to complementary -donor–acceptor interactions between porphyrin and m-DNB, which resulted in the improvement of response and sensitivity. These experimental data suggest that electrochemical response on porphyrin functionalized CNTs film was superior to CNTs, though the electron transfer rate of CNTs was higher than porphyrin functionalized CNTs film. These results are in accordance with our hypothesis that CNTs was a crucial matrix to form grid porphyrin film and the introduction of porphyrin to CNTs improved the electrochemical response of explosives. Furthermore, the peak currents of MHTPP/CNTs/GCE were linearly increased with scan rates in the range of 20–120 mV/s, demonstrating a surface confined redox process. The signal (−0.598 V), which exhibits the most favorable characteristic, was selected for subsequent analytical work. Fig. 4 showed the effect of pH on the CNTs-porphyrin/catalytic system. Curve (c) has a sloping background current and the electrochemical response is faint. Comparatively, the background current vanished and two characteristic peaks of 1, 3-dinitrobenzene observed obviously with the increasing of pH (curve a, b). The origin of the response of porphyrin/CNTs film that is consistent with pH dependencies may attribute to the fact that H+ in the solution could form electrostatic interaction with electron cloud of benzene ring and oxygen of nitryl in m-DNB molecules [22]. Increasing pH facilitated deprotonation of remaining functional groups of CNTs and gave higher charge densities [23], which strengthened the preconcentration capability of CNTs to concentrate more m-DNB at the surface of film.
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Fig. 4. pH effect on response for 1 M m-DNB (a) pH 7 PBS contained 0.1 M NaCl, (b) pH 10 PBS contained 0.1 M NaCl, (c) pH 6 PBS contained 0.1 M NaCl.
3.4. Differential pulse stripping voltammetry at the MHTPP/CNTs/GCE Under optimized condition, DPSV was exploited to investigate the response of m-DNB due to its potential-controlled preconcentration at the surface of functionalized film [24]. Calibration curves were presented in the inset of Fig. 5, in which the proposed sensor exhibited good linear response ranging from 8.0 × 10−9 to 5.0 × 10−7 mol L−1 with correlation coefficients of 0.99854 and 0.99569 (for n ( 10) for peak −0.598 V and peak −0.7 V respectively and with detection limit of ca. 2.0 × 10−9 mol L−1 (signal to noise ratio of 3). The result based on this method was compared with other methods reported about explosives detection in solution as listed in Table 1. It is noted that this method with a simple preparation provides better linear range and detection limit. 3.5. Reproducibility and stability The reproducibility of the analytical response which obtained from six different electrodes constructed by the same procedure was analyzed. The relative standard deviation (RSD) was 2.3%
Fig. 5. DPSV of m-DNB at MHTPP/CNTs/GCE in 0.1 M NaCl pH 10 under the optimum condition. From bottom to top concentration: 0, 0.008, 0.03, 0.05, 0.08, 0.1, 0.175, 0.24, 0.29, 0.42, 0.5 M and the inset shows calibration plot (a) peak I at −0.524 V, (b) peak II at −0.640 V.
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Table 1 Line range and detection limit for different explosives sensors. Different explosive sensors Mesoporous carbon material IL-GNs MIP/MWCNT MIMs-AuNPs-GCE Porphyrin/CNTs film
Line range (mol L−1 ) −8
5.0 × 10 3.0 × 10−8 4.5 × 10−8 4.0 × 10−8 8.0 × 10−9
−5
to 2.1 × 10 to 1.5 × 10−6 to 8.5 × 10−6 to 3.2 × 10−6 to 5.0 × 10−7
Detection limit (mol L−1 )
Reference
2.5 × 10−8 4.0 × 10−9 2.5 × 10−8 1.3 × 10−8 2.0 × 10−9
[25] [26] [27] [28] This paper
IL–GNs: ionic liquid-graphene hybrid nanosheets; MIP: molecularly imprinted polymers; MIMs: molecular imprinting monolayers.
(n = 6). Moreover, the current response retained 98.2% of its initial current sensitivity after the electrode stored at indoor condition for one week, demonstrating a good stability. 3.6. The mechanism of interaction between porphyrin film and explosives To confirm the mechanism of interaction between porphyrin film and explosives, we designed a novel type of porphyrin film, namely meso-tetrakis (4-hydroxyphenyl)porphyrin (THTPP), in which the para position of each benzene ring was substituted with electro-rich hydroxyl group. The electron-rich hydroxyl group interact with the electron-deficient nitro group of explosives to form acid-base paring firstly, which is similar to the affinity binding of explosive molecules to electron-rich 3aminopropyltriethoxysilane (APTS)-modified alumnia pore walls [29]. And then the interaction between the -macrocycle of porphyrin and the benzen ring take place by a parallel-planar to form strong electron donor–acceptor (EDA) complexes as present in Scheme 2. We anticipated that the additional hydroxyl group could provide more opportunities to form hydrogen bond with nitro group of explosive and the -donor–accepter interaction between porphyrin and explosive should be enhanced by the additional hydroxyl group. Thus we presumed that the electrochemical response of THTPP functionalized CNTs film was higher than
Fig. 6. Electrochemical response of THPP functionalized CNTs film (b) and MHTPP functionalized CNTs film (a) (A) in 1, 3-dinitrobenzene, (B) in parachloronitrobenzene.
MHTPP functionalized CNTs film. As expected, the electrochemical responses of THTPP film are dramatically enhanced by the compared to that of the MHTPP film (Fig. 6). The results demonstrated that the electrochemical response was enhanced by increasing the number of –OH substitution in porphyrin. This information also provides the platform for a practical strategy for rational design of the sensor of explosives. 4. Conclusion
Scheme 2. Schematic representation of the interaction between porphyrin and mDNB. (A) The hydroxyl group on benzene ring of porphyrin interacted with nitryl in NACs molecules firstly, (B) the electron donor–acceptor (EDA) complexes between porphyrin and m-DNB secondly.
In conclusion, we report herein a novel electrochemical sensor for explosives based on porphyrin functionalized CNTs film. The study demonstrates that the attachment of porphyrin on CNTs led to more receptor sites to explosive compound and the synergetic effect of porphyrin and CNTs induced porphyrin/CNTs as a better sensor for explosive compound. On one hand, CNTs utilize the endemic preconcentration capability to accumulate explosive compound. On the other hand, -donor–acceptor complexes take
X. Lu et al. / Colloids and Surfaces B: Biointerfaces 88 (2011) 396–401
place at porphyrin film. It has been shown that in the formation grid porphyrin film, carbon nanotubes acted as the crucial base materials. Most of important, the results also showed that the electrochemical response was enhanced by increasing the number of –OH substitution in porphyrin. On the basis of this designing strategy, it should be possible to develop a novel class of explosives probe based on porphyrin and CNTs.
[7] [8] [9] [10] [11] [12] [13]
Acknowledgements This work was supported by the National Natural Science Foundation of China (no. 20927004, 20775060, 20875077), the Science and Technology Support Projects of Gansu Province (no. 1011GKCA025), the Natural Science Foundation of Gansu Province (no. 0701RJZA109, no. 0803RJZA105 and no. 096RJZA122) and Key Laboratory of Polymer Materials of Gansu Province, China. References [1] R.A. Larson, P.L. Miller, T.O. Crowley, Environ. Sci. Technol. 30 (1996) 1192. [2] R.C. Stringer, S. Gangopadhyay, S.A. Grant, Anal. Chem. 82 (2010) 4015. [3] N.L. Sanders, S. Kothari, G. Huang, G. Salazar, R.G. Cooks, Anal. Chem. 82 (2010) 5313. [4] A. Bromberg, R.A. Mathies, Anal. Chem. 75 (2003) 1188. [5] C. Yu, J. Guo, H. Gu, Electroanal., 9999 (2010). [6] H.X. Zhang, Q. Chen, R. Wen, J.S. Hu, L.J. Wan, Anal. Chem. 79 (2007) 2179.
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Contents lists available at ScienceDirect
Colloids and Surfaces B: Biointerfaces journal homepage: www.elsevier.com/locate/colsurfb
Determination of explosives based on novel type of sensor using porphyrin functionalized carbon nanotubes Xiaoquan Lu ∗ , Yanli Quan, Zhonghua Xue, Bowan Wu, Hetong Qi, Dong Liu Key Laboratory of Bioelectrochemistry and Environmental Analysis of Gansu Province, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China
a r t i c l e
i n f o
Article history: Received 27 February 2011 Received in revised form 28 May 2011 Accepted 6 July 2011 Available online 18 July 2011 Keywords: Porphyrin Carbon nanotubes Synergetic effect Explosive Interaction
a b s t r a c t The hydroxide of meso-tetraphenylporphyrin derivatives functionalized carbon nanotubes (CNTs) was fabricated in our research to explore the interaction between porphyrin and explosive. It was turned out that in the formation of grid porphyrin film, carbon nanotubes as a cruciul base materials promoted the electron transfer rate. Most of important, the results also showed that the electrochemical response was enhanced through increasing the number of –OH substitution in porphyrin. Such information provides the platform for a practical strategy for rational design of the sensor of explosives. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Nitroaromatic compounds, which are commonly used in the manufacture of explosives, pesticides, dyes and pharmaceuticals, have drawn considerable attention to environmental monitoring. Due to the high toxicity, wide distribution, and stable chemical character [1], the rapid and precise detection of these toxic agents in the environment become increasingly important for homeland security and health protection. To our knowledge, various strategies, including fluorescence [2], spectrometer [3], microfabricated capillary electrophoresis chip [4], etc., have been conducted on explosives. Electrochemical methods seem to be excellent candidates for the rapid detection of explosives owing to their simple preparation and easier to fabricate portable detectors [5]. Recently, kinds of electrochemical sensors for detecting explosives have been explored. It was reported that attempts have been made in search of superior -electron donors to interact with nitroaromatic compounds by strong – interactions [6], or utilize novel materials [7]. Carbon nanotubes (CNTs) have been examined for the removal of contaminants particularly those having aromatic -systems from water and gases attributed to their large external surface area [8]. Recently, various studies using CNTs as adsorbents have been carried out by some groups [7,9]. However, just rely on the preconcentration capability of CNTs to detect explosives is far
∗ Corresponding author. Tel.: +86 931 7971276; fax: +86 931 7971323. E-mail address: [email protected] (X. Lu). 0927-7765/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfb.2011.07.020
from enough. Therefore, it is necessary to employ more-compact and low-cost method to monitor explosives. Porphyrin compounds are widely employed as platform to recognize various species because of their structures with high delocalized -bonds and the unique properties of the tetraphenylporphyrin core which has ability to participate in hydrogen bonds with the anion to form the hydrogen bond donor–accepor complexes [10]. Their electrochemical and photophysical properties make porphyrin derivatives combine with a complementary electron-accepting species to mimic the electron transport of biological process based on the formation of electron donor/acceptor (D/A). W. Kutner has reported that porphyrin was able to interact with fulleren to form the supramolecular donor–acceptor hybrid, in which the acceptor was coordinated to the surface-accessible porphyrin sites [11]. Besides, due to the strong electron-withdrawing ability of the nitro group, explosives have the ability to accept electron from electron donor to form electron donor–acceptor (EDA) complexes, which involves electron transferred from the highest occupied molecular orbital (HOMO) of donor to the lowest unoccupied molecular orbital (LUMO) of acceptor unpaired electrons [12,13]. Considering porphyrin as a typical -donor compound, we envisioned the introduction of the hydroxyphenyl-porphyrin into CNTs could enhance the sensitivity of the detection of explosives based on the synergetic effects of porphyrin and CNTs (Scheme 1). It is supposed that the mechanism involved in this strategy as follows: on one hand, CNTs utilize the endemic preconcentration capability to accumulate explosives, on the other hand, the electron-deficient
X. Lu et al. / Colloids and Surfaces B: Biointerfaces 88 (2011) 396–401
397
Scheme 1. Schematic representation of electrochemical sensing explosives. (A) The CNTs to glass carbon electrode surface; (B) the electropolymerization porphyrin to CNTs surface; (C) explosive adsorbs to the functionalized film; (D) electrochemical stripping detection of explosive.
nitro group of explosives interact with the electron-rich hydroxyl group of porphyrin to form acid-base paring interaction instantaneously and then the aromatic ring of explosive turn into planar and interact with the -macrocycle of porphyrin by a parallelplanar to form strong donor–acceptor (EDA) complexes, which involves electron transfer from -donor of porphyrin to electrondeficient explosive. In order to confirm the validity of this approach and the rationality of this hypothesis, we initiated a study aimed at providing a strategy for rational design of electrochemical sensor based on the 5-(4-hydroxyphenyl)-10,15,20-triphenylporphyrin (MHTPP) functionalized CNTs film. To provide further insight into the interaction between explosives and porphyrin, the CNTs was also fuctionalized by meso-tetrakis(4-hydroxyphenyl)porphyrin (THTPP) to probe the electrochemical response of explosives. 2. Experiment 2.1. Apparatus and reagents Stock solutions of m-DNB (1000 mg/L) (purchased from Beijing Chemical Co. Ltd.) were dissolved by acetonitrile (ACN). Analyte solutions were prepared by dilution the stock solutions using PBS contained 0.1 M NaCl and protected from light. Electrochemical studies were performed with CHI660 electrochemical work station (CHI, USA) using conventional three-electrode cells with bare and modified GC electrode as work electrodes, a platinum wire counter electrode, and Ag/AgCl reference electrode. The surface morphologies of the prepared electrodes were observed through scanning electron microscopy (SEM) on JSM-6701F (Japan Electron Optics Company). UV1102 spectrometer (Techcomp Bio-Equipment LTD, Shanghai, China) was used to obtain UV–vis spectra. 2.2. Preparation of MHTPP/CNTs/GCE The commercially available CNTs was treated to induce carboxylic acid groups on the surface [14]. The treated CNTs was dispersed in 2 mL N, N-dimethylformamide (DMF) with the aid of ultrasonic agitation to give 1 mg/mL black suspension. Glassy carbon electrode (GCE, 2 mm in diameter) was carefully polished and rinsed completely by distilled water. Afterwards, 5 L suspension was cast onto the surface of GCE pretreated and then dried under infrared lamp. Finally, CNTs/GCE was cleaned by ethanol and distilled water respectively. The porphyrin film was accomplished by the application of 8 potential cycles from acetonitrile and 0.1 mM porphyrin containing 0.02 M tetrabutylammonium perchlorate (TBAP) solutions as electrolyte solution at a scan rate of
50 mV/s ranging from 0.3 V to 1.2 V versus Ag/AgCl [15,16]. After covalent functionalized, the electrode was cleaned by acetonitrile and distilled water respectively to exclude residual monomer from the electrode. 3. Results and discussion 3.1. The formation of porphyrin film The available redox chemistry for hydroxyphenylporphyrins shows their dual functionality: porphyrin and polyphenol. Unprotonated nitrogen in the central region of porphyrin ring is negative charge which can lose one electron to form pyrrole radical cation, and react with carboxyl of electrode surface [15]. It is believed that the hydroxyl of benzene ring carried some unpaired electron could cause the p- conjugation between hydroxy and conjugated benzene ring, which makes the phenyl carbon atom at the meta position become radical cation and thus the nucleophilic attack of adjacent porphyrin hydroxy group takes place at its phenyl carbon atom at the meta position with respect to the OH group [17,16]. Such nucleophilic attack resulted in an ether linkage between the adjacent porphyrins at the meta carbon of the phenyl ring [18]. During the oxidative polymerization process formed a dimer-like radical-ion species and thus the peak appeared at 0.9 V in the present of porphyrin was attributed to the formation of radical-ion species. As can be seen from Fig. 1, compared with cyclic voltammograms in acetonitrile and electrolyte solution in the absence of porphyrin (Fig. 1a), the current at 0.9 V decreases upon continued electropolymerization in the present of porphyrin, indicating passivated polymeric film formed on the electrode blocking the access of the monomer to the electrode surface (Fig. 1b). However, the growth behavior on CNTs substrate (Fig. 1c) was different from that on the flat GC substrate. At about 1.08 V emerges additional peak may ascribed to self-aggregate forming poly-phenoxy selfaggregated dimer films. Furthermore, the current response at 0.9 V shifts to slight negative during electropolymerization, indicating CNTs result in the formation of porphyrin film more easily. 3.2. Characterization of different modified electrodes The scanning electron micrographs of purified CNTs and MHTPP/CNTs are showed in Fig. 2 to confirm the formation and growth of polymer film on the CNTs. The nanocomposite displayed porous structures composed of cemented entangled network structure (Fig. 2A), which is different from the structure of the purified CNTs (Fig. 2B), indicating thin layer porphyrin film
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Fig. 1. (a) Cyclic voltammograms of bare electrode in acetonitrile containing 0.02 M TBAP in the absence of porphyrin in a sequence of 8 cyclic scans and at a scan rate of 50 mV/s, (b) Cyclic voltammograms of glass carbon electrode in acetonitrile containing 0.1 mM porphyrin and 0.02 M TBAP. (c) Cyclic voltammograms of CNTs fuctionalized electrode during electropolymerization process in acetonitrile containing 0.1 mM porphyrin and 0.02 M TBAP.
electropolymerized onto CNTs. The UV–vis absorption spectrum of MHTPP, MHTPP/CNTs, and CNTs are presented in Fig. 2C. The Soret band of MHTPP/CNTs (curve b, Fig. 2C) splits into two obvious absorption band in comparison with that of the solution profile (curve a, Fig. 2C). The spectral changes result from the fact that the deprotonation of the phenolic protons increased the energy
of orbitals localized on the phenoxide anion substituents [19] and caused a charge-transfer transition from phenoxide anion to porphyrin further to the surface of CNTs and thus resulted in the aggregation of porphyrin [13,20]. Cyclic voltammetry (CV) was performed to anticipate the surface properties of different films using the redox couple Fe(CN)6 3−/4− as redox probe to identify the electron transfer efficiency. The electrochemical response was suppressed to the lowest value as MHTPP functionalized glassy carbon electrode (curve a, Fig. 2D) compared with bare glassy carbon electrode (curve b, Fig. 2D). This result demonstrates that the electron transfer was inhibited by dense nonconductive porphyrin film in which the porphyrin rings may be dense packed with a parallel orientation to the bare carbon glass electrode. Nevertheless, CNTs with large surface area as matrix (curve c, Fig. 2D), the electron transfer efficiency is promoted and analyte cross ultrathin membrane of porphyrin became more easily compared with MHTPP/GCE. Nonetheless, it should be noted that the electrochemical response was lower than CNTs/GCE (curve d, Fig. 2D) indicating the presence of porphyrin film retard electron transfer to the redox probe at the CNTs/solution interface. These results also indicate that porous CNTs film with large surface area provides ideal matrix for the distribution of porphyrin and promotes the electron-transfer reactions. So we have speculated that in the formation grid porphyrin film, carbon nanotubes acted as the crucial base materials to promote the electronic transfer rate. 3.3. Electrocatalytic detection of m-DNB and the optimization of analytical condition To verify our hypothesis of the beginning, we investigate the electrochemical responses of different modified films to the 1,
Fig. 2. (A) Scanning electron microscopy (SEM) image of MHTPP/CNTs/GCE (B) SEM image of CNTs/GCE (C) Soret band region of (a) MHTPP, (b) MHTPP/CNTs, (c) CNTs in 8:2 ethanol/acetonitrile (D) CVs of (a) MHTPP/GCE, (b) GCE, (c) MHTPP/CNTs/GCE, (d) CNTs/GCE in 5 mM Fe(CN)6 3−/4− containing 0.1 M KCl.
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Fig. 3. CVs (a) GCE, (b) MHTPP/GCE, (c) CNTs/GCE, (d) MHTPP/CNTs/GCE of pH 10 contained 0.5 mM m-DNB at scan rate 50 mV/s.
3-dinitrobenzene. Fig. 3 reveals the electrochemical response of different modified films to the m-DNB. The cathodic peaks at −0.598 V and −0.714 V correspond to the stepwise reduction of nitro groups to the amine groups through the nitroso and hydroxylamine intermediates, according to the mechanism for the reduction of nitro groups [7]. Compared with the weak electrochemical response of bare electrode (Fig. 3a), CNTs/GCE (Fig. 3c) showed better reduction peaks associated with endemic preconcentration capability of CNTs to concentrate hydrophobic organic compounds at the surface of CNTs [21]. Fig. 3b indicated that compact insulated porphyrin film prevented the explosives from reaching the surface of electrode and reduced the possibility of interaction. In contrast, the response of peak current increased when CNTs acted as matrix to functionalized porphyrin (Fig. 3d), which reveals that CNTs could increase the surface area and create interstice spaces which serve as m-DNB conducting channels. Meanwhile, the attachment of porphyrin ultrathin film on CNTs provides more receptor sites to explosive compound and leads to complementary -donor–acceptor interactions between porphyrin and m-DNB, which resulted in the improvement of response and sensitivity. These experimental data suggest that electrochemical response on porphyrin functionalized CNTs film was superior to CNTs, though the electron transfer rate of CNTs was higher than porphyrin functionalized CNTs film. These results are in accordance with our hypothesis that CNTs was a crucial matrix to form grid porphyrin film and the introduction of porphyrin to CNTs improved the electrochemical response of explosives. Furthermore, the peak currents of MHTPP/CNTs/GCE were linearly increased with scan rates in the range of 20–120 mV/s, demonstrating a surface confined redox process. The signal (−0.598 V), which exhibits the most favorable characteristic, was selected for subsequent analytical work. Fig. 4 showed the effect of pH on the CNTs-porphyrin/catalytic system. Curve (c) has a sloping background current and the electrochemical response is faint. Comparatively, the background current vanished and two characteristic peaks of 1, 3-dinitrobenzene observed obviously with the increasing of pH (curve a, b). The origin of the response of porphyrin/CNTs film that is consistent with pH dependencies may attribute to the fact that H+ in the solution could form electrostatic interaction with electron cloud of benzene ring and oxygen of nitryl in m-DNB molecules [22]. Increasing pH facilitated deprotonation of remaining functional groups of CNTs and gave higher charge densities [23], which strengthened the preconcentration capability of CNTs to concentrate more m-DNB at the surface of film.
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Fig. 4. pH effect on response for 1 M m-DNB (a) pH 7 PBS contained 0.1 M NaCl, (b) pH 10 PBS contained 0.1 M NaCl, (c) pH 6 PBS contained 0.1 M NaCl.
3.4. Differential pulse stripping voltammetry at the MHTPP/CNTs/GCE Under optimized condition, DPSV was exploited to investigate the response of m-DNB due to its potential-controlled preconcentration at the surface of functionalized film [24]. Calibration curves were presented in the inset of Fig. 5, in which the proposed sensor exhibited good linear response ranging from 8.0 × 10−9 to 5.0 × 10−7 mol L−1 with correlation coefficients of 0.99854 and 0.99569 (for n ( 10) for peak −0.598 V and peak −0.7 V respectively and with detection limit of ca. 2.0 × 10−9 mol L−1 (signal to noise ratio of 3). The result based on this method was compared with other methods reported about explosives detection in solution as listed in Table 1. It is noted that this method with a simple preparation provides better linear range and detection limit. 3.5. Reproducibility and stability The reproducibility of the analytical response which obtained from six different electrodes constructed by the same procedure was analyzed. The relative standard deviation (RSD) was 2.3%
Fig. 5. DPSV of m-DNB at MHTPP/CNTs/GCE in 0.1 M NaCl pH 10 under the optimum condition. From bottom to top concentration: 0, 0.008, 0.03, 0.05, 0.08, 0.1, 0.175, 0.24, 0.29, 0.42, 0.5 M and the inset shows calibration plot (a) peak I at −0.524 V, (b) peak II at −0.640 V.
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Table 1 Line range and detection limit for different explosives sensors. Different explosive sensors Mesoporous carbon material IL-GNs MIP/MWCNT MIMs-AuNPs-GCE Porphyrin/CNTs film
Line range (mol L−1 ) −8
5.0 × 10 3.0 × 10−8 4.5 × 10−8 4.0 × 10−8 8.0 × 10−9
−5
to 2.1 × 10 to 1.5 × 10−6 to 8.5 × 10−6 to 3.2 × 10−6 to 5.0 × 10−7
Detection limit (mol L−1 )
Reference
2.5 × 10−8 4.0 × 10−9 2.5 × 10−8 1.3 × 10−8 2.0 × 10−9
[25] [26] [27] [28] This paper
IL–GNs: ionic liquid-graphene hybrid nanosheets; MIP: molecularly imprinted polymers; MIMs: molecular imprinting monolayers.
(n = 6). Moreover, the current response retained 98.2% of its initial current sensitivity after the electrode stored at indoor condition for one week, demonstrating a good stability. 3.6. The mechanism of interaction between porphyrin film and explosives To confirm the mechanism of interaction between porphyrin film and explosives, we designed a novel type of porphyrin film, namely meso-tetrakis (4-hydroxyphenyl)porphyrin (THTPP), in which the para position of each benzene ring was substituted with electro-rich hydroxyl group. The electron-rich hydroxyl group interact with the electron-deficient nitro group of explosives to form acid-base paring firstly, which is similar to the affinity binding of explosive molecules to electron-rich 3aminopropyltriethoxysilane (APTS)-modified alumnia pore walls [29]. And then the interaction between the -macrocycle of porphyrin and the benzen ring take place by a parallel-planar to form strong electron donor–acceptor (EDA) complexes as present in Scheme 2. We anticipated that the additional hydroxyl group could provide more opportunities to form hydrogen bond with nitro group of explosive and the -donor–accepter interaction between porphyrin and explosive should be enhanced by the additional hydroxyl group. Thus we presumed that the electrochemical response of THTPP functionalized CNTs film was higher than
Fig. 6. Electrochemical response of THPP functionalized CNTs film (b) and MHTPP functionalized CNTs film (a) (A) in 1, 3-dinitrobenzene, (B) in parachloronitrobenzene.
MHTPP functionalized CNTs film. As expected, the electrochemical responses of THTPP film are dramatically enhanced by the compared to that of the MHTPP film (Fig. 6). The results demonstrated that the electrochemical response was enhanced by increasing the number of –OH substitution in porphyrin. This information also provides the platform for a practical strategy for rational design of the sensor of explosives. 4. Conclusion
Scheme 2. Schematic representation of the interaction between porphyrin and mDNB. (A) The hydroxyl group on benzene ring of porphyrin interacted with nitryl in NACs molecules firstly, (B) the electron donor–acceptor (EDA) complexes between porphyrin and m-DNB secondly.
In conclusion, we report herein a novel electrochemical sensor for explosives based on porphyrin functionalized CNTs film. The study demonstrates that the attachment of porphyrin on CNTs led to more receptor sites to explosive compound and the synergetic effect of porphyrin and CNTs induced porphyrin/CNTs as a better sensor for explosive compound. On one hand, CNTs utilize the endemic preconcentration capability to accumulate explosive compound. On the other hand, -donor–acceptor complexes take
X. Lu et al. / Colloids and Surfaces B: Biointerfaces 88 (2011) 396–401
place at porphyrin film. It has been shown that in the formation grid porphyrin film, carbon nanotubes acted as the crucial base materials. Most of important, the results also showed that the electrochemical response was enhanced by increasing the number of –OH substitution in porphyrin. On the basis of this designing strategy, it should be possible to develop a novel class of explosives probe based on porphyrin and CNTs.
[7] [8] [9] [10] [11] [12] [13]
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