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Electrospun nanofibers of porphyrinated polyimide for the ultra-sensitive detection of trace TNT
Sensors and Actuators B 184 (2013) 205–211
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Sensors and Actuators B: Chemical journal homepage: www.elsevier.com/locate/snb
Electrospun nanofibers of porphyrinated polyimide for the ultra-sensitive detection of trace TNT Yuan-Yuan Lv a,c,d , Wei Xu a,c , Fu-Wen Lin a,c , Jian Wu a,c,∗ , Zhi-Kang Xu b,c,∗∗ a
Department of Chemistry, Zhejiang University, Hangzhou 310027, China MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China Cyrus Tang Center for Sensor Materials and Applications, Zhejiang University, Hangzhou 310027, China d School of Medicine, Zhejiang University City College, Hangzhou 310015, China b
c
a r t i c l e
i n f o
Article history: Received 13 February 2013 Received in revised form 12 April 2013 Accepted 19 April 2013 Available online 28 April 2013 Keywords: Electrospinning Nanofibers Porphyrin Explosive detection TNT
a b s t r a c t Nanofibers with distinct luminescent property were facilely fabricated by electrospinning from porphyrinated polyimide and demonstrated as a kind of novel sensory material for trace detection of TNT vapor (10 ppb). Covalently bonding of porphyrin fluorophores into the polyimide main chains reduces the aggregation-caused fluorescence self-quenching of porphyrin and improves the physicochemical stability of the polyimide nanofibers. The large surface area-to-volume ratio and hence good gas accessibility endow the porphyrinated nanofibers with much more remarkable fluorescent quenching behavior toward trace TNT vapor than its spin-coating dense film counterparter. Besides TNT, 2,4-dinitrotoluene (DNT), 2,4,6-trinitrophenol (PA) and nitrobenzene (NB) could also quench the fluorescence of the porphyrinated nanofibers, but the quenching efficiency is much lower than that of TNT. An apparent binding affinity constant of (2.37 ± 0.19) × 107 L/mol was calculated from SPR analysis, confirming that the porphyrinated nanofibers is a promising alternative for TNT detection. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, much attention has been paid to the development of novel materials for explosives detection, due to the needs in public security, industrial process control, and environmental protection [1,2]. It is well known 2,4,6-trinitrotoluene trinitrotoluene (TNT), with vapor pressure of 5.8 × 10−6 Torr (∼10 ppb) at 25 ◦ C, is a leading example of explosives which always correlate with criminal terrorist attacks that constrain our everyday life [3]. Therefore, the detection of TNT in a fast, simple, sensitive, reliable, and cost-effective manner is extremely important for practical applications ranging from minefield remediation to crime scene investigations and counter-terrorism activities [1,4]. Compared with various chemical or physical methods have been developed for this purpose, fluorescence-based chemosensing attracts much attention due to its high sensitivity and simplicity [5–18]. A TNT molecule has a low-energy unoccupied * orbital, which can accept an electron from the excited state of electron
∗ Corresponding author at: Department of Chemistry, Zhejiang University, Hangzhou 310027, China. Fax: +86 571 87951773. ∗∗ Corresponding author at: MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China. Fax: +86 571 87951773. E-mail addresses: [email protected] (J. Wu), [email protected] (Z.-K. Xu). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.04.094
donors. This specific molecular structure makes the detection of TNT on the basis of electron-transfer luminescence quenching achievable [5–22]. A series of fluorescent conjugated polymers have been developed as TNT sensory materials [8–18]. However, it seems that the multi-step processes for the synthesis of conjugated polymers restrict their practical applications in somewhat. Among various sensory materials, porphyrin and its metallic derivatives consist of a fundamental component for building sensing probes due to their excellent molar absorptivity and fluorescence quantum yields [23–25]. Fluorescence quenching of porphyrin normally result from the well-known – interactions between -electron-rich porphyrin chromophores and -electronpoor analytes (such as TNT), which makes the porphyrin-based materials feasible and promising for TNT detection [26,27]. Since the vapor pressure of TNT in the ambiance is extremely low, great efforts have been made in order to improve the detection sensitivity of porphyrin-based sensing materials. Electrospinning is a simple and versatile technique for generating ultralong and continuous nano- to microscale fibers. The electrospun fibers possess a number of characteristics such as high specific surface area, high aspect ratio, and unique porosity which are beneficial for improving sensitivity and response time in sensing applications [28]. Li and co-workers synthesized electrospun silica nanofibers modified with porphyrin for the detection of explosives, the large surface area-to-volume ratio of the obtained
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nanofibrous membranes were found to be principally responsible for the remarkable sensing performance toward TNT [20]. However, further results are needed to demonstrate the physicochemical stability and reusability of these porphyrin-doped materials for consecutively sensing cycles. In our previous work, a series of porphyrin-containing polymers were synthesized and electrospun into nanofibers with well preservation of fluorescent properties and porous nanostructures [29–31]. Kawakami and co-workers also reported a porphyrin-containing electrospun polyacrylonitrile nanofibers and studied the possibility of the positional control of porphyrin molecules in the electrospun polymer nanofibers for catalytic applications [32]. However, the chemical and mechanical stability of the porphyrinated polyacrylonitriles nanofibers is not matched for the detection of TNT. Polyimide, with superior chemical stabilities as well as mechanical properties, is an excellent polymer to overcome this obstacle [30,31,33,34]. In this work, we synthesized zinc porphyrinated polyimide (ZnPPI) and electrospun it into nanofibers for the ultra-sensitive detection of TNT vapor. Scheme 1 illustrates the molecular structure of ZnPPI and the process of porphyrinated nanofibers for fluorescent detection of TNT. Quantitative analysis was carried out with surface plasmon resonance (SPR) as an insight into the affinity interactions between the porphyrinated nanofibers and TNT at molecular level. Adsorption isotherms were constructed to determine the ultra sensitivity of TNT binding to the porphyrinated nanofibers. As a matter of fact, introducing porphyrin moieties into polyimide backbones avoids possible bleaching of the chromophores, physicochemical instability, and undesired fluorescence quenching caused by porphyrin aggregation, which are major disadvantages counteracting their application as sensory materials [35,36]. 2. Experimental 2.1. Materials Zinc 5,10-bis(4-aminophenyl)-15,20-diphenylporphyrin (cisZnDATPP) was synthesized using a reported procedure [37,38]. 4,4 Hexafluoroisopropylidenediphthalic anhydride (6FDA) (reagent grade, 99%) was purchased from Aldrich and dried for 6 h at 160 ◦ C prior to use. Oxydianiline (ODA) was commercially obtained from Shanghai Chemical Agent Co. (China) and purified through sublimation above its gasification temperature before use. N,N Dimethyl acetamide (DMAc, analytical reagent grade, purchased from Shanghai Chemical Agent Co., China), was dried over 4 A˚ molecular sieves for the following use. All the other reagents were of analytical degree and used as received without further purification, unless noted otherwise. 2.2. Synthesis of the porphyrinated polyimide (ZnPPI) Diamino-monomers, namely cis-ZnDATPP (0.129 g, 0.2 mmol) and ODA (0.16 mg, 0.8 mmol), were dissolved in dry DMAc (10 mL). 6FDA (0.444 g, 1.0 mmol) was then added slowly with vigorously stirring. The mixture was stirred at 0 ◦ C for 2 h, and then at 25 ◦ C for another 16 h in a N2 atmosphere. The resulting poly(amic acid) was subject to chemical imidization by adding acetic anhydride and triethylamine of the same amount with stirring at room temperature for 24 h. Then, the polyimide solution was transferred into ethanol, and the crimson, fibrous precipitate was collected by filtration. Further purification of the polyimide was accomplished by thoroughly washing with methanol prior to being dried under reduced pressure at 60 ◦ C for 2 days. ZnPPI was finally obtained in 89% yield. Polyimide (PI) from 6FDA and ODA (mole ratios – 1:1) without
porphyrin moieties was also synthesized in the same fashion for thermogravimetric analyses comparison. 2.3. Electrospinning Typical procedure for the electrospinning of porphyrinated nanofibers is as follows [30,31,39]. ZnPPI was dissolved in DMAc to form a 15 wt.% solution. The resulting viscous solution was pumped into a syringe through a metal needle at a constant rate of 1.0 mL/h by a microinfusion pump (WZ-50C2, Zhejiang University Medical Instrument Co., Ltd., China). Application of a high voltage (16 kV) to the metal syringe needle allowed the generation of nanofibers which were collected on the surface of a grounded aluminum plate (distance from the syringe needle to the plate was 12 cm). Thus obtained nanofibers were detached from the aluminum foil and dried at 60 ◦ C to remove residual solvent. 2.4. Characterization The intrinsic viscosity of the resultant ZnPPI was measured at a concentration of 0.5 g/dL in DMAc at 30 ◦ C. FT-IR spectra were recorded on a spectrometer (Nicolet, Nexus-470, USA) with the accessories of attenuated total reflectance (ATR). 1 H NMR spectra were measured in DMSO-d6 on a Bruker (Advance DMX500) NMR spectrometer. UV–vis spectra were recorded in a quartz cell on a Shimadzu UV 2450 spectrophotometer (Shimadzu, Japan). UV–vis spectra in solid state were obtained using a Shimadzu integrating sphere assembly attached to the above-mentioned spectrophotometer. Fluorescence emission spectrum was conducted on Shimadzu RF-5310 PC fluorescence spectrophotometer with a solid assembly to have excitation and emission at 45◦ to the membrane surface. Excited at 420 nm, samples emissions were measured at wavelength of 600–800 nm. Thermogravimetric analyses (TGA) were employed at a heating rate of 10 ◦ C/min in N2 with a NETZSCH STA 409 PC/PG thermogravimetric analyzer. SEM image was taken on a Field Emission SEM (SIRION, FEI, USA) after the sample was spurted with Au for 7 min to make it conductive. All fluorescence images were taken by the use of confocal laser scanning microscopy (CLSM) with a Leica TCS SP5 confocal setup mounted on a Leica DMI 6000 CS inverted microscope (Leica Microsystems, Germany) and was operated under the Leica Application Suite Advanced Fluorescence (LASAF) program, in which the wavelength of excitation is 488 nm. 2.5. Fluorescence quenching study of the porphyrinated nanofibers The fluorescence response of the porphyrinated nanofibers to TNT and some other vapors of nitro-containing aromatics, 2,4dinitrotoluene (DNT), 2,4,6-trinitrophenol (PA) and nitrobenzene (NB) was studied in a similar way that Swager and his co-workers described [8–11]. Specially, a sealed vial (10 mL) was filled with a small amount of solid explosive or cotton wetted by NB liquid, and set over night to ensure saturation vapor pressure had been reached (TNT (10 ppb), DNT (180 ppb) PA (7.7 × 10−3 ppb) and NB (3 × 105 ppb)). Exposure of a sample of the porphyrinated nanofibers to the vapors of explosives was performed by placing a sensing membrane into the sealed vial at room temperature (25 ◦ C). After exposing for a given period of time, the sensing sample was taken out and fluorescence spectra were measured immediately at excitation wavelength of 420 nm. The sample was set at a fixed position to reduce the signal variation and every fluorescence sensing process were taken three times to reduce the deviation thus greatly improve the reliability of the data.
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Scheme 1. Molecular structure of porphyrinated polyimide and the schematic illustrations of the porphyrinated nanofibrous membrane for fluorescent detection of TNT.
2.6. SPR analyses for interactions between porphyrinated polyimide and TNT Both the SPR gold chips and quartz crystal microbalance (QCM) gold-coated AT cut quartz crystal chips were cleaned by dipping in ethanol for 10 min, in freshly made piranha solution (concentrated H2 SO4 and 30% H2 O2 in proportion of 3:1) for 1 min, followed by extensively rinsing with ultra-pure water (18.2 M cm). Then all the chips were dried in nitrogen gas. Porphyrinated nanofibers were electrospun on SPR and QCM chips using the same electrospinning method mentioned above. As a comparison, dense film was fabricated by depositing a 2.5 wt.% ZnPPI solution on SPR/QCM chips at a spinning rate of 2000 rpm for 20 s then dried for 24 h at 60 ◦ C under reduced pressure. Both porphyrinated nanofibers and dense film formed on QCM chips were used to quantify the different coating load on SPR chips using quartz crystal microbalance (QCM) at 25 ◦ C (QTZ, Resonance Probe GmbH, Goslar, Germany). The calculated results revealed that the density of porphyrinated nanofibers and dense film coated on SPR chips were 2.17 g/cm2 and 6.98 g/cm2 , respectively [39]. Certain concentrations of TNT solution was injected into the Reichert SR7100 DC instrument (Reichert, USA) and flowed over sensor surface at a rate of 25 L/min. Ultra-pure water took a role as the buffer solution during the whole analysis process. Temperature was rigorously controlled at 25.0 ± 0.1 ◦ C throughout the experiments.
3. Results and discussion 3.1. Synthesis and characterization of the porphyrin-containing polyimide The chemical structure of porphyrinated polyimide is shown in Scheme 1 and the synthesis route is similar to the common polyimides. Zinc 5,10-bis(4-aminophenyl)-15,20-diphenylporphyrin (cis-ZnDATPP) was copolymerized into polyimide backbones with 4,4 -oxidianiline (ODA) and 4,4 -hexafluoroisopropylidenediphthalic anhydride (6FDA) to obtain the porphyrinated polyimide (ZnPPI). Herein, cis-ZnDATPP was chosen due to its relative higher yield and quantum yield than its isomeric compound, zinc 5,15bis (4-aminophenyl)-10,20-diphenylporphyrin (trans-ZnDATPP). It was found that the molar ratios of cis-ZnDATPP to ODA in the synthesized ZnPPI is 0.23 (calculated from 1 H NMR spectrum in Supporting Information, Fig. S1), which is very close to 0.25 (molar ratios of cis-ZnDATPP to ODA in feed for ZnPPI synthesis) and
the inherent viscosity of the ZnPPI is 0.558 dL/g. Furthermore, its fluorescence quantum yields (˚F ) in DMAc was determined to be 0.19 taking tetraphenylporphyrin (TPP) as standard (˚ref = 0.15) [40] and using the method described by Demas and Crosby [41]. 3.2. Fluorescence response characteristics of ZnPPI to TNT in solution As for TNT-sensing application, it is necessary and instructive to evaluate the fluorescence response behavior of ZnPPI toward TNT in solution firstly. It can be seen from Fig. 1 that the fluorescence emission intensity of ZnPPI in solution decreases gradually with the increase of TNT concentration, which suggests that ZnPPI exhibits a certain extent of sensitivity toward TNT. 3.3. Preparation and characterization of nanofibers from ZnPPI Porphyrinated nanofibers were successfully electrospun from a 15 wt.% ZnPPI solution in DMAc. FESEM micrographs (Fig. 2A) indicate that the nanofibers have a smooth and uniform morphology with a diameter of around 261 ± 27 nm. FT-IR/ATR spectrum shows characteristic absorption bands for polyimide at 1775, 1725, 1380 and 725 cm−1 , respectively (Supporting Information, Fig. S2). Thermogravimetric curve (Supporting Information, Fig. S3) indicates that the porphyrinated nanofibers preserves the thermalresistant nature of PI synthesized from 6FDA and ODA (mole ratios – 1:1) without porphyrin moieties, for which 5% weight loss happens at approximately 500 ◦ C. Fig. 3A displays typical absorption spectrum of the porphyrinated nanofibers. Characteristic transitions can be seen for zinc porphyrin with an intense porphyrin
Fig. 1. Fluorescence responses of the porphyrinated polyimide solution (0.6 mg/mL) as a function of increasing amount of TNT in DMAc excited at 365 nm using a UV lamp.
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Fig. 2. FESEM micrograph (A), and CLSM images of the porphyrinated nanofibrous membrane before (B) or after (C) exposing to 10 ppb TNT vapor (ex = 488 nm).
Soret band at 420 nm and two weak Q-bands at 564 and 605 nm, respectively. The relative intensity of each peak is similar to that of ZnPPI in solution. In agreement with this result, the porphyrinated nanofibers show a distinct fluorescence emission spectrum of porphyrin when excited at 420 nm (Fig. 3B). However, the emission wavelength of the nanofibers is slightly red-shifted from the solution. Similar phenomena have been observed previously for many TNT-relative sensory materials, and have been partly attributed to the -stacking interactions between polymer backbones in solid state [8,14,17,24]. It is well known that the aggregation of porphyrin chromophores causes fluorescence quenching, which detrimentally affects the application of corresponding materials [35,36]. However, results presented here reveal that this phenomenon can be effectively inhibited by chemically separating the chromophoric units into the polyimide backbones. Similar effect has been reported in our previous work when porphyrins were attached onto polyacrylonitrile chain as pendant groups [29]. Taking these results together, we can conclude that the incorporation of chromophores either into polymer backbones or as pendant groups is a good choice to preserve their fluorescence properties. Besides, electrospinning the porphyrinated polyimide into nanofibers is another key factor significantly avoiding the – stacking interactions of the chromophores [17]. Red light emits uniformly from the nanofibers, as can be seen from Fig. 2B.
3.4. Sensing performance of the porphyrinated nanofibers to trace vapors of explosives Fig. 4 shows the time-dependent fluorescence emission spectra of the nanofibers upon exposure to saturated TNT vapor (10 ppb) at room temperature. The intensity decreases continuously with an increase in the exposing time. It is clear that only 10 s exposure causes ∼60% reduction of the fluorescence emission, and 100 s exposure results in nearly fully fluorescence quenching of the porphyrinated nanofibers, which is also demonstrated by CLSM images (Fig. 2B and C). This quenching efficiency is comparable to or much higher than those previously reported fluorescencebased film sensors [7–9,16–18]. For example, it was reported that pentiptycene-derived phenyleneethynylene polymers [9], monolayer assembly of oligo(diphenylsilane) film [18] and cadmium porphyrin-doped mesoporous silica film [19] showed 30%, 55% and 56% reduction of the fluorescence emission after 10 s of exposure to TNT vapor (10 ppb), respectively. A possible explanation for this feature is the electron-transfer donor–acceptor mechanism [21,42,43]. A strong driving force is provided for the fast fluorescence quenching of zinc porphyrin moieties due to the well-known – stacking interactions as well as adequate energy level matching between zinc porphyrin and TNT. In addition, the unique porous structure of the nonwoven nanofibers is considered to be another
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key factor in enhancing the fluorescence quenching efficiency [28,44–47]. Fig. 4 indicates that the emission intensity at 646 nm is nearly reverted back to its original form when the quenched nanofibers were puffed with nitrogen gas. Further examination was conducted to demonstrate the reusability of these nanofibers. As illustrated in Fig. 5, fluorescence of the nanofibers remains nearly unchanged after five times of quenching and regeneration. This fact proves the sensing process is basically reversible. The morphology of sensing materials may have some effects on sensitivity as mentioned above. Therefore, the fluorescence emission spectrum of dense films fabricated by spin-coating was also measured for comparison. Clearly, the nanofibers exhibit four times higher response (expressed as a quenching efficiency of fluorescence intensity) than the dense film fabricated from the same porphyrinated polyimide (Fig. 6). This can be ascribed to the three dimensional and porous structures of the nonwoven nanofibers, and hence good gas accessibility, as well as more porphyrin moieties locating at or near the fiber surface. It was found that besides TNT, some other vapors of nitrocontaining aromatics, DNT, NB and PA could also quench the
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fluorescence of the porphyrinated nanofibers, but the quenching efficiency is much lower than that of TNT. Fig. 7 illustrates the time-dependence of the fluorescence intensity of the porphyrinated nanofibers exposed to different analytes. For those explosive vapors, TNT, DNT, NB and PA, the quenching efficiencies are in the order of TNT > DNT > NB > PA. Theoretic studies have shown that the fluorescence quenching efficiency affects by various factors, mainly including the vapor pressure of the analyte, the exergonicity of electron transfer reaction between sensing dye and analytes, and the binding strength (sensing dye–analyte interaction) [8,9]. The studied nitro aromatic compounds are all of -electron-poor analytes. All of them can quench the fluorescence of the porphyrinated nanofibers due to the electron transfer between -electron-rich porphyrin chromophores and these analytes. However, it is different for the fluorescence quenching efficiency. The binding strength of TNT and PA to porphyrin ring is higher than that of DNT and NB because of their extra electron-withdrawing nitro group. Although the vapor pressures of DNT and NB are much higher than TNT, the fluorescence quenching efficiency of them is still lower than TNT. As for PA, the lowest quenching efficiency can be attributed to its much lower vapor pressure. These results indicate that, for different explosives (TNT, DNT, NB and PA), the porphyrinated nanofibers show
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Fig. 5. Fluorescent recovery curve of the porphyrinated nanofibers. Quenching time: 5 min; regenerated by puffing with N2 for 10 min.
Fig. 7. Time-dependent fluorescence quenching efficiency of the porphyrinated nanofibers for different analytes (saturated vapors, except 30 ppm for HCl).
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on their availability in both gaseous and aqueous media for TNT detection. 4. Conclusions Porphyrinated polyimide nanofibers can be facilely fabricated for the ultra-sensitive detection of TNT. The notable fluorescence quenching to TNT vapor precisely meets the requirements outlined by novel fluorescent sensing materials. SPR analysis confirms the distinct sensitivity for these nanofibers toward TNT in solution. This promising result comes from the combining advantages of polymer nanofibers and porphyrin. It provides a platform for other porphyrinated nanofibers as various sensing materials when considering the large versality of porphyrin in polymer synthesis.
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Acknowledgments
Fig. 8. SPR response induced by the porphyrinated nanofibrous membrane (solid line) and dense film (dashed line) deposited on SPR chips when exposed to TNT solution with different concentrations. Average density of the porphyrinated nanofibrous membrane and dense film coated on SPR chips are 2.17 g cm−2 and 6.98 g cm−2 , respectively. And H2 O is the buffer solution throughout the whole process.
The authors gratefully acknowledge the financial support from the National Science Foundation of China (Grant No. 50973094). We also thank Dr. Ling-Shu Wan, Dr. Xiao-Jun Huang, Mr. Hai-Yang Wang and Dr. Zhen-Mei Liu for their discussion at the beginning of this work.
different quenching behaviors which can be potentially used to distinguish different kinds of explosives [12–16,18–22]. Moreover, no significant change in fluorescence emission was observed upon exposing the nanofibers to the vapors of common organic solvents and daily chemicals, such as saturated toluene, acetone vapors and 30 ppm of gaseous HCl.
Appendix A. Supplementary data
3.5. Stoichiometric analysis of TNT sensing properties at molecular level
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Surface plasmon resonance (SPR) was used to further study the affinity interactions between the porphyrinated nanofibers and TNT. Fig. 8 shows the typical SPR responses of the nanofibers deposited on SPR chip to TNT aqueous solution with different concentrations. Increasing TNT concentration leads to progressive increase of the SPR response. The nanofibers were then regenerated by rinsing with ultra-pure water. The binding of TNT to the porphyrinated nanofibers was analyzed with the Langmuir adsorption model by the following equation [48]: C C 1 1 = + · R Rmax Rmax KA
(1)
where C is the concentration of injected TNT, R is the SPR response variation before and after injection of TNT aqueous solution, Rmax is the SPR response variation when C is infinity, and KA is the apparent binding affinity constant. KA between the porphyrinated nanofibers and TNT is found to be (2.37 ± 0.19) × 107 L/mol, indicating the ZnPPI nanofibers has great potentiality as a sensory material for TNT. Sensitivity comparison was also carried out from SPR analyses of the porphyrinated nanofibers and dense film with different coating loads determined by QCM. In our cases, the mass of the nanofibers on a SPR chip is nearly 1/3 of the corresponding dense film. For a TNT solution with certain concentration, however, the nanofibers exhibit much larger angle shift than the dense film. In particular, TNT solution with ultra-low concentration (5 ppb) excites much more obvious SPR signal from the nanofibers compared with the neglectable signal from the dense film. This may be explained by the fact that the nanofibers with porous structures have 1–2 orders of magnitude more surface area than that of the dense film [28,49–53]. As a consequence, the porphyrinated nanofibers with high surface area has ultra-sensitivity to TNT. Furthermore, it seems that the porphyrinated nanofibers shed lights
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2013.04.094. References
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Biographies Yuan-Yuan Lv received her Ph.D. degree in Chemistry from Zhejiang University, China in 2010. She started working in the School of Medicine, Zhejiang University City College since 2010. Her research interests are centered on functional polymers and nanostructured materials with optical and electrical characteristics for chemical sensors. Wei Xu received his B.S. degree in Chemistry from Anhui University, China in 2012. He is currently a postgraduate student in Department of Chemistry, Zhejiang University, China. His research interests are centered on polymer and nanostructured materials for sensors. Fu-Wen Lin graduated from Nanjing University of Technology, China in 2010. Presently, he is a Ph.D. candidate in Department of Chemistry, Zhejiang University, China. His research interests are porphyrin and corresponding polymers. Dr. Jian Wu is a full professor in Department of Chemistry, Zhejiang University, China. Her research interests are porphyrin and corresponding polymers. Dr. Zhi-Kang Xu is a full professor in Department of Polymer Science and Engineering, Zhejiang University, China. He is currently a member of editorial boards of Journal of Membrane Science. His research interests include surface engineering of polymer membranes, polymeric materials for bio- and chemicosensors, and nanostructured materials.
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Electrospun nanofibers of porphyrinated polyimide for the ultra-sensitive detection of trace TNT Yuan-Yuan Lv a,c,d , Wei Xu a,c , Fu-Wen Lin a,c , Jian Wu a,c,∗ , Zhi-Kang Xu b,c,∗∗ a
Department of Chemistry, Zhejiang University, Hangzhou 310027, China MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China Cyrus Tang Center for Sensor Materials and Applications, Zhejiang University, Hangzhou 310027, China d School of Medicine, Zhejiang University City College, Hangzhou 310015, China b
c
a r t i c l e
i n f o
Article history: Received 13 February 2013 Received in revised form 12 April 2013 Accepted 19 April 2013 Available online 28 April 2013 Keywords: Electrospinning Nanofibers Porphyrin Explosive detection TNT
a b s t r a c t Nanofibers with distinct luminescent property were facilely fabricated by electrospinning from porphyrinated polyimide and demonstrated as a kind of novel sensory material for trace detection of TNT vapor (10 ppb). Covalently bonding of porphyrin fluorophores into the polyimide main chains reduces the aggregation-caused fluorescence self-quenching of porphyrin and improves the physicochemical stability of the polyimide nanofibers. The large surface area-to-volume ratio and hence good gas accessibility endow the porphyrinated nanofibers with much more remarkable fluorescent quenching behavior toward trace TNT vapor than its spin-coating dense film counterparter. Besides TNT, 2,4-dinitrotoluene (DNT), 2,4,6-trinitrophenol (PA) and nitrobenzene (NB) could also quench the fluorescence of the porphyrinated nanofibers, but the quenching efficiency is much lower than that of TNT. An apparent binding affinity constant of (2.37 ± 0.19) × 107 L/mol was calculated from SPR analysis, confirming that the porphyrinated nanofibers is a promising alternative for TNT detection. © 2013 Elsevier B.V. All rights reserved.
1. Introduction In recent years, much attention has been paid to the development of novel materials for explosives detection, due to the needs in public security, industrial process control, and environmental protection [1,2]. It is well known 2,4,6-trinitrotoluene trinitrotoluene (TNT), with vapor pressure of 5.8 × 10−6 Torr (∼10 ppb) at 25 ◦ C, is a leading example of explosives which always correlate with criminal terrorist attacks that constrain our everyday life [3]. Therefore, the detection of TNT in a fast, simple, sensitive, reliable, and cost-effective manner is extremely important for practical applications ranging from minefield remediation to crime scene investigations and counter-terrorism activities [1,4]. Compared with various chemical or physical methods have been developed for this purpose, fluorescence-based chemosensing attracts much attention due to its high sensitivity and simplicity [5–18]. A TNT molecule has a low-energy unoccupied * orbital, which can accept an electron from the excited state of electron
∗ Corresponding author at: Department of Chemistry, Zhejiang University, Hangzhou 310027, China. Fax: +86 571 87951773. ∗∗ Corresponding author at: MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Zhejiang University, Hangzhou 310027, China. Fax: +86 571 87951773. E-mail addresses: [email protected] (J. Wu), [email protected] (Z.-K. Xu). 0925-4005/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.snb.2013.04.094
donors. This specific molecular structure makes the detection of TNT on the basis of electron-transfer luminescence quenching achievable [5–22]. A series of fluorescent conjugated polymers have been developed as TNT sensory materials [8–18]. However, it seems that the multi-step processes for the synthesis of conjugated polymers restrict their practical applications in somewhat. Among various sensory materials, porphyrin and its metallic derivatives consist of a fundamental component for building sensing probes due to their excellent molar absorptivity and fluorescence quantum yields [23–25]. Fluorescence quenching of porphyrin normally result from the well-known – interactions between -electron-rich porphyrin chromophores and -electronpoor analytes (such as TNT), which makes the porphyrin-based materials feasible and promising for TNT detection [26,27]. Since the vapor pressure of TNT in the ambiance is extremely low, great efforts have been made in order to improve the detection sensitivity of porphyrin-based sensing materials. Electrospinning is a simple and versatile technique for generating ultralong and continuous nano- to microscale fibers. The electrospun fibers possess a number of characteristics such as high specific surface area, high aspect ratio, and unique porosity which are beneficial for improving sensitivity and response time in sensing applications [28]. Li and co-workers synthesized electrospun silica nanofibers modified with porphyrin for the detection of explosives, the large surface area-to-volume ratio of the obtained
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nanofibrous membranes were found to be principally responsible for the remarkable sensing performance toward TNT [20]. However, further results are needed to demonstrate the physicochemical stability and reusability of these porphyrin-doped materials for consecutively sensing cycles. In our previous work, a series of porphyrin-containing polymers were synthesized and electrospun into nanofibers with well preservation of fluorescent properties and porous nanostructures [29–31]. Kawakami and co-workers also reported a porphyrin-containing electrospun polyacrylonitrile nanofibers and studied the possibility of the positional control of porphyrin molecules in the electrospun polymer nanofibers for catalytic applications [32]. However, the chemical and mechanical stability of the porphyrinated polyacrylonitriles nanofibers is not matched for the detection of TNT. Polyimide, with superior chemical stabilities as well as mechanical properties, is an excellent polymer to overcome this obstacle [30,31,33,34]. In this work, we synthesized zinc porphyrinated polyimide (ZnPPI) and electrospun it into nanofibers for the ultra-sensitive detection of TNT vapor. Scheme 1 illustrates the molecular structure of ZnPPI and the process of porphyrinated nanofibers for fluorescent detection of TNT. Quantitative analysis was carried out with surface plasmon resonance (SPR) as an insight into the affinity interactions between the porphyrinated nanofibers and TNT at molecular level. Adsorption isotherms were constructed to determine the ultra sensitivity of TNT binding to the porphyrinated nanofibers. As a matter of fact, introducing porphyrin moieties into polyimide backbones avoids possible bleaching of the chromophores, physicochemical instability, and undesired fluorescence quenching caused by porphyrin aggregation, which are major disadvantages counteracting their application as sensory materials [35,36]. 2. Experimental 2.1. Materials Zinc 5,10-bis(4-aminophenyl)-15,20-diphenylporphyrin (cisZnDATPP) was synthesized using a reported procedure [37,38]. 4,4 Hexafluoroisopropylidenediphthalic anhydride (6FDA) (reagent grade, 99%) was purchased from Aldrich and dried for 6 h at 160 ◦ C prior to use. Oxydianiline (ODA) was commercially obtained from Shanghai Chemical Agent Co. (China) and purified through sublimation above its gasification temperature before use. N,N Dimethyl acetamide (DMAc, analytical reagent grade, purchased from Shanghai Chemical Agent Co., China), was dried over 4 A˚ molecular sieves for the following use. All the other reagents were of analytical degree and used as received without further purification, unless noted otherwise. 2.2. Synthesis of the porphyrinated polyimide (ZnPPI) Diamino-monomers, namely cis-ZnDATPP (0.129 g, 0.2 mmol) and ODA (0.16 mg, 0.8 mmol), were dissolved in dry DMAc (10 mL). 6FDA (0.444 g, 1.0 mmol) was then added slowly with vigorously stirring. The mixture was stirred at 0 ◦ C for 2 h, and then at 25 ◦ C for another 16 h in a N2 atmosphere. The resulting poly(amic acid) was subject to chemical imidization by adding acetic anhydride and triethylamine of the same amount with stirring at room temperature for 24 h. Then, the polyimide solution was transferred into ethanol, and the crimson, fibrous precipitate was collected by filtration. Further purification of the polyimide was accomplished by thoroughly washing with methanol prior to being dried under reduced pressure at 60 ◦ C for 2 days. ZnPPI was finally obtained in 89% yield. Polyimide (PI) from 6FDA and ODA (mole ratios – 1:1) without
porphyrin moieties was also synthesized in the same fashion for thermogravimetric analyses comparison. 2.3. Electrospinning Typical procedure for the electrospinning of porphyrinated nanofibers is as follows [30,31,39]. ZnPPI was dissolved in DMAc to form a 15 wt.% solution. The resulting viscous solution was pumped into a syringe through a metal needle at a constant rate of 1.0 mL/h by a microinfusion pump (WZ-50C2, Zhejiang University Medical Instrument Co., Ltd., China). Application of a high voltage (16 kV) to the metal syringe needle allowed the generation of nanofibers which were collected on the surface of a grounded aluminum plate (distance from the syringe needle to the plate was 12 cm). Thus obtained nanofibers were detached from the aluminum foil and dried at 60 ◦ C to remove residual solvent. 2.4. Characterization The intrinsic viscosity of the resultant ZnPPI was measured at a concentration of 0.5 g/dL in DMAc at 30 ◦ C. FT-IR spectra were recorded on a spectrometer (Nicolet, Nexus-470, USA) with the accessories of attenuated total reflectance (ATR). 1 H NMR spectra were measured in DMSO-d6 on a Bruker (Advance DMX500) NMR spectrometer. UV–vis spectra were recorded in a quartz cell on a Shimadzu UV 2450 spectrophotometer (Shimadzu, Japan). UV–vis spectra in solid state were obtained using a Shimadzu integrating sphere assembly attached to the above-mentioned spectrophotometer. Fluorescence emission spectrum was conducted on Shimadzu RF-5310 PC fluorescence spectrophotometer with a solid assembly to have excitation and emission at 45◦ to the membrane surface. Excited at 420 nm, samples emissions were measured at wavelength of 600–800 nm. Thermogravimetric analyses (TGA) were employed at a heating rate of 10 ◦ C/min in N2 with a NETZSCH STA 409 PC/PG thermogravimetric analyzer. SEM image was taken on a Field Emission SEM (SIRION, FEI, USA) after the sample was spurted with Au for 7 min to make it conductive. All fluorescence images were taken by the use of confocal laser scanning microscopy (CLSM) with a Leica TCS SP5 confocal setup mounted on a Leica DMI 6000 CS inverted microscope (Leica Microsystems, Germany) and was operated under the Leica Application Suite Advanced Fluorescence (LASAF) program, in which the wavelength of excitation is 488 nm. 2.5. Fluorescence quenching study of the porphyrinated nanofibers The fluorescence response of the porphyrinated nanofibers to TNT and some other vapors of nitro-containing aromatics, 2,4dinitrotoluene (DNT), 2,4,6-trinitrophenol (PA) and nitrobenzene (NB) was studied in a similar way that Swager and his co-workers described [8–11]. Specially, a sealed vial (10 mL) was filled with a small amount of solid explosive or cotton wetted by NB liquid, and set over night to ensure saturation vapor pressure had been reached (TNT (10 ppb), DNT (180 ppb) PA (7.7 × 10−3 ppb) and NB (3 × 105 ppb)). Exposure of a sample of the porphyrinated nanofibers to the vapors of explosives was performed by placing a sensing membrane into the sealed vial at room temperature (25 ◦ C). After exposing for a given period of time, the sensing sample was taken out and fluorescence spectra were measured immediately at excitation wavelength of 420 nm. The sample was set at a fixed position to reduce the signal variation and every fluorescence sensing process were taken three times to reduce the deviation thus greatly improve the reliability of the data.
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Scheme 1. Molecular structure of porphyrinated polyimide and the schematic illustrations of the porphyrinated nanofibrous membrane for fluorescent detection of TNT.
2.6. SPR analyses for interactions between porphyrinated polyimide and TNT Both the SPR gold chips and quartz crystal microbalance (QCM) gold-coated AT cut quartz crystal chips were cleaned by dipping in ethanol for 10 min, in freshly made piranha solution (concentrated H2 SO4 and 30% H2 O2 in proportion of 3:1) for 1 min, followed by extensively rinsing with ultra-pure water (18.2 M cm). Then all the chips were dried in nitrogen gas. Porphyrinated nanofibers were electrospun on SPR and QCM chips using the same electrospinning method mentioned above. As a comparison, dense film was fabricated by depositing a 2.5 wt.% ZnPPI solution on SPR/QCM chips at a spinning rate of 2000 rpm for 20 s then dried for 24 h at 60 ◦ C under reduced pressure. Both porphyrinated nanofibers and dense film formed on QCM chips were used to quantify the different coating load on SPR chips using quartz crystal microbalance (QCM) at 25 ◦ C (QTZ, Resonance Probe GmbH, Goslar, Germany). The calculated results revealed that the density of porphyrinated nanofibers and dense film coated on SPR chips were 2.17 g/cm2 and 6.98 g/cm2 , respectively [39]. Certain concentrations of TNT solution was injected into the Reichert SR7100 DC instrument (Reichert, USA) and flowed over sensor surface at a rate of 25 L/min. Ultra-pure water took a role as the buffer solution during the whole analysis process. Temperature was rigorously controlled at 25.0 ± 0.1 ◦ C throughout the experiments.
3. Results and discussion 3.1. Synthesis and characterization of the porphyrin-containing polyimide The chemical structure of porphyrinated polyimide is shown in Scheme 1 and the synthesis route is similar to the common polyimides. Zinc 5,10-bis(4-aminophenyl)-15,20-diphenylporphyrin (cis-ZnDATPP) was copolymerized into polyimide backbones with 4,4 -oxidianiline (ODA) and 4,4 -hexafluoroisopropylidenediphthalic anhydride (6FDA) to obtain the porphyrinated polyimide (ZnPPI). Herein, cis-ZnDATPP was chosen due to its relative higher yield and quantum yield than its isomeric compound, zinc 5,15bis (4-aminophenyl)-10,20-diphenylporphyrin (trans-ZnDATPP). It was found that the molar ratios of cis-ZnDATPP to ODA in the synthesized ZnPPI is 0.23 (calculated from 1 H NMR spectrum in Supporting Information, Fig. S1), which is very close to 0.25 (molar ratios of cis-ZnDATPP to ODA in feed for ZnPPI synthesis) and
the inherent viscosity of the ZnPPI is 0.558 dL/g. Furthermore, its fluorescence quantum yields (˚F ) in DMAc was determined to be 0.19 taking tetraphenylporphyrin (TPP) as standard (˚ref = 0.15) [40] and using the method described by Demas and Crosby [41]. 3.2. Fluorescence response characteristics of ZnPPI to TNT in solution As for TNT-sensing application, it is necessary and instructive to evaluate the fluorescence response behavior of ZnPPI toward TNT in solution firstly. It can be seen from Fig. 1 that the fluorescence emission intensity of ZnPPI in solution decreases gradually with the increase of TNT concentration, which suggests that ZnPPI exhibits a certain extent of sensitivity toward TNT. 3.3. Preparation and characterization of nanofibers from ZnPPI Porphyrinated nanofibers were successfully electrospun from a 15 wt.% ZnPPI solution in DMAc. FESEM micrographs (Fig. 2A) indicate that the nanofibers have a smooth and uniform morphology with a diameter of around 261 ± 27 nm. FT-IR/ATR spectrum shows characteristic absorption bands for polyimide at 1775, 1725, 1380 and 725 cm−1 , respectively (Supporting Information, Fig. S2). Thermogravimetric curve (Supporting Information, Fig. S3) indicates that the porphyrinated nanofibers preserves the thermalresistant nature of PI synthesized from 6FDA and ODA (mole ratios – 1:1) without porphyrin moieties, for which 5% weight loss happens at approximately 500 ◦ C. Fig. 3A displays typical absorption spectrum of the porphyrinated nanofibers. Characteristic transitions can be seen for zinc porphyrin with an intense porphyrin
Fig. 1. Fluorescence responses of the porphyrinated polyimide solution (0.6 mg/mL) as a function of increasing amount of TNT in DMAc excited at 365 nm using a UV lamp.
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Fig. 2. FESEM micrograph (A), and CLSM images of the porphyrinated nanofibrous membrane before (B) or after (C) exposing to 10 ppb TNT vapor (ex = 488 nm).
Soret band at 420 nm and two weak Q-bands at 564 and 605 nm, respectively. The relative intensity of each peak is similar to that of ZnPPI in solution. In agreement with this result, the porphyrinated nanofibers show a distinct fluorescence emission spectrum of porphyrin when excited at 420 nm (Fig. 3B). However, the emission wavelength of the nanofibers is slightly red-shifted from the solution. Similar phenomena have been observed previously for many TNT-relative sensory materials, and have been partly attributed to the -stacking interactions between polymer backbones in solid state [8,14,17,24]. It is well known that the aggregation of porphyrin chromophores causes fluorescence quenching, which detrimentally affects the application of corresponding materials [35,36]. However, results presented here reveal that this phenomenon can be effectively inhibited by chemically separating the chromophoric units into the polyimide backbones. Similar effect has been reported in our previous work when porphyrins were attached onto polyacrylonitrile chain as pendant groups [29]. Taking these results together, we can conclude that the incorporation of chromophores either into polymer backbones or as pendant groups is a good choice to preserve their fluorescence properties. Besides, electrospinning the porphyrinated polyimide into nanofibers is another key factor significantly avoiding the – stacking interactions of the chromophores [17]. Red light emits uniformly from the nanofibers, as can be seen from Fig. 2B.
3.4. Sensing performance of the porphyrinated nanofibers to trace vapors of explosives Fig. 4 shows the time-dependent fluorescence emission spectra of the nanofibers upon exposure to saturated TNT vapor (10 ppb) at room temperature. The intensity decreases continuously with an increase in the exposing time. It is clear that only 10 s exposure causes ∼60% reduction of the fluorescence emission, and 100 s exposure results in nearly fully fluorescence quenching of the porphyrinated nanofibers, which is also demonstrated by CLSM images (Fig. 2B and C). This quenching efficiency is comparable to or much higher than those previously reported fluorescencebased film sensors [7–9,16–18]. For example, it was reported that pentiptycene-derived phenyleneethynylene polymers [9], monolayer assembly of oligo(diphenylsilane) film [18] and cadmium porphyrin-doped mesoporous silica film [19] showed 30%, 55% and 56% reduction of the fluorescence emission after 10 s of exposure to TNT vapor (10 ppb), respectively. A possible explanation for this feature is the electron-transfer donor–acceptor mechanism [21,42,43]. A strong driving force is provided for the fast fluorescence quenching of zinc porphyrin moieties due to the well-known – stacking interactions as well as adequate energy level matching between zinc porphyrin and TNT. In addition, the unique porous structure of the nonwoven nanofibers is considered to be another
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Fig. 3. Absorption (A) and fluorescence emission (B) spectra of the porphyrinated nanofibrous membrane (solid line) and porphyrinated polyimide in DMAc solution (dashed line).
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Wavelength (nm) Fig. 4. Time-dependent fluorescence emission spectra of the porphyrinated nanofibrous membrane upon exposure to 10 ppb of TNT vapor (room temperature) at 0, 10, 30, 60, 180, 300, and 600 s (top to bottom, solid lines), and recovered when puffing with N2 for certain time (dashed line).
key factor in enhancing the fluorescence quenching efficiency [28,44–47]. Fig. 4 indicates that the emission intensity at 646 nm is nearly reverted back to its original form when the quenched nanofibers were puffed with nitrogen gas. Further examination was conducted to demonstrate the reusability of these nanofibers. As illustrated in Fig. 5, fluorescence of the nanofibers remains nearly unchanged after five times of quenching and regeneration. This fact proves the sensing process is basically reversible. The morphology of sensing materials may have some effects on sensitivity as mentioned above. Therefore, the fluorescence emission spectrum of dense films fabricated by spin-coating was also measured for comparison. Clearly, the nanofibers exhibit four times higher response (expressed as a quenching efficiency of fluorescence intensity) than the dense film fabricated from the same porphyrinated polyimide (Fig. 6). This can be ascribed to the three dimensional and porous structures of the nonwoven nanofibers, and hence good gas accessibility, as well as more porphyrin moieties locating at or near the fiber surface. It was found that besides TNT, some other vapors of nitrocontaining aromatics, DNT, NB and PA could also quench the
0
100
200
300
400
500
600
700
Time (s) Fig. 6. Time-dependent fluorescence quenching efficiency of the porphyrinated nanofibrous membrane () and dense film () upon exposure to TNT vapor (10 ppb).
fluorescence of the porphyrinated nanofibers, but the quenching efficiency is much lower than that of TNT. Fig. 7 illustrates the time-dependence of the fluorescence intensity of the porphyrinated nanofibers exposed to different analytes. For those explosive vapors, TNT, DNT, NB and PA, the quenching efficiencies are in the order of TNT > DNT > NB > PA. Theoretic studies have shown that the fluorescence quenching efficiency affects by various factors, mainly including the vapor pressure of the analyte, the exergonicity of electron transfer reaction between sensing dye and analytes, and the binding strength (sensing dye–analyte interaction) [8,9]. The studied nitro aromatic compounds are all of -electron-poor analytes. All of them can quench the fluorescence of the porphyrinated nanofibers due to the electron transfer between -electron-rich porphyrin chromophores and these analytes. However, it is different for the fluorescence quenching efficiency. The binding strength of TNT and PA to porphyrin ring is higher than that of DNT and NB because of their extra electron-withdrawing nitro group. Although the vapor pressures of DNT and NB are much higher than TNT, the fluorescence quenching efficiency of them is still lower than TNT. As for PA, the lowest quenching efficiency can be attributed to its much lower vapor pressure. These results indicate that, for different explosives (TNT, DNT, NB and PA), the porphyrinated nanofibers show
Flourescence Intensity (arbitrary units)
TNT vapor
N2 1
2
3 Cycles
4
5
Fig. 5. Fluorescent recovery curve of the porphyrinated nanofibers. Quenching time: 5 min; regenerated by puffing with N2 for 10 min.
Fig. 7. Time-dependent fluorescence quenching efficiency of the porphyrinated nanofibers for different analytes (saturated vapors, except 30 ppm for HCl).
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on their availability in both gaseous and aqueous media for TNT detection. 4. Conclusions Porphyrinated polyimide nanofibers can be facilely fabricated for the ultra-sensitive detection of TNT. The notable fluorescence quenching to TNT vapor precisely meets the requirements outlined by novel fluorescent sensing materials. SPR analysis confirms the distinct sensitivity for these nanofibers toward TNT in solution. This promising result comes from the combining advantages of polymer nanofibers and porphyrin. It provides a platform for other porphyrinated nanofibers as various sensing materials when considering the large versality of porphyrin in polymer synthesis.
pp b 10 0
pp b 70
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pp b
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Acknowledgments
Fig. 8. SPR response induced by the porphyrinated nanofibrous membrane (solid line) and dense film (dashed line) deposited on SPR chips when exposed to TNT solution with different concentrations. Average density of the porphyrinated nanofibrous membrane and dense film coated on SPR chips are 2.17 g cm−2 and 6.98 g cm−2 , respectively. And H2 O is the buffer solution throughout the whole process.
The authors gratefully acknowledge the financial support from the National Science Foundation of China (Grant No. 50973094). We also thank Dr. Ling-Shu Wan, Dr. Xiao-Jun Huang, Mr. Hai-Yang Wang and Dr. Zhen-Mei Liu for their discussion at the beginning of this work.
different quenching behaviors which can be potentially used to distinguish different kinds of explosives [12–16,18–22]. Moreover, no significant change in fluorescence emission was observed upon exposing the nanofibers to the vapors of common organic solvents and daily chemicals, such as saturated toluene, acetone vapors and 30 ppm of gaseous HCl.
Appendix A. Supplementary data
3.5. Stoichiometric analysis of TNT sensing properties at molecular level
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Surface plasmon resonance (SPR) was used to further study the affinity interactions between the porphyrinated nanofibers and TNT. Fig. 8 shows the typical SPR responses of the nanofibers deposited on SPR chip to TNT aqueous solution with different concentrations. Increasing TNT concentration leads to progressive increase of the SPR response. The nanofibers were then regenerated by rinsing with ultra-pure water. The binding of TNT to the porphyrinated nanofibers was analyzed with the Langmuir adsorption model by the following equation [48]: C C 1 1 = + · R Rmax Rmax KA
(1)
where C is the concentration of injected TNT, R is the SPR response variation before and after injection of TNT aqueous solution, Rmax is the SPR response variation when C is infinity, and KA is the apparent binding affinity constant. KA between the porphyrinated nanofibers and TNT is found to be (2.37 ± 0.19) × 107 L/mol, indicating the ZnPPI nanofibers has great potentiality as a sensory material for TNT. Sensitivity comparison was also carried out from SPR analyses of the porphyrinated nanofibers and dense film with different coating loads determined by QCM. In our cases, the mass of the nanofibers on a SPR chip is nearly 1/3 of the corresponding dense film. For a TNT solution with certain concentration, however, the nanofibers exhibit much larger angle shift than the dense film. In particular, TNT solution with ultra-low concentration (5 ppb) excites much more obvious SPR signal from the nanofibers compared with the neglectable signal from the dense film. This may be explained by the fact that the nanofibers with porous structures have 1–2 orders of magnitude more surface area than that of the dense film [28,49–53]. As a consequence, the porphyrinated nanofibers with high surface area has ultra-sensitivity to TNT. Furthermore, it seems that the porphyrinated nanofibers shed lights
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.snb.2013.04.094. References
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Biographies Yuan-Yuan Lv received her Ph.D. degree in Chemistry from Zhejiang University, China in 2010. She started working in the School of Medicine, Zhejiang University City College since 2010. Her research interests are centered on functional polymers and nanostructured materials with optical and electrical characteristics for chemical sensors. Wei Xu received his B.S. degree in Chemistry from Anhui University, China in 2012. He is currently a postgraduate student in Department of Chemistry, Zhejiang University, China. His research interests are centered on polymer and nanostructured materials for sensors. Fu-Wen Lin graduated from Nanjing University of Technology, China in 2010. Presently, he is a Ph.D. candidate in Department of Chemistry, Zhejiang University, China. His research interests are porphyrin and corresponding polymers. Dr. Jian Wu is a full professor in Department of Chemistry, Zhejiang University, China. Her research interests are porphyrin and corresponding polymers. Dr. Zhi-Kang Xu is a full professor in Department of Polymer Science and Engineering, Zhejiang University, China. He is currently a member of editorial boards of Journal of Membrane Science. His research interests include surface engineering of polymer membranes, polymeric materials for bio- and chemicosensors, and nanostructured materials.