Fluorescent chemosensor for the detection of histamine based on dendritic porphyrin-incorporated nanofibers

European Polymer Journal 48 (2012) 1988–1996

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European Polymer Journal journal homepage: www.elsevier.com/locate/europolj

Macromolecular Nanotechnology

Fluorescent chemosensor for the detection of histamine based on dendritic porphyrin-incorporated nanofibers Da Young Seong a, Myung-Seok Choi b, Young-Jin Kim a,⇑ a b

Department of Biomedical Engineering, Catholic University of Daegu, Gyeongsan 712-702, Republic of Korea Department of Materials Chemistry and Engineering, Konkuk University, Seoul 143-701, Republic of Korea

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a r t i c l e

i n f o

Article history: Received 24 May 2012 Received in revised form 11 September 2012 Accepted 16 September 2012 Available online 25 September 2012 Keywords: Nanofiber Dendritic porphyrin Fluorescent chemosensor Histamine PCL

a b s t r a c t To use as a fluorescent chemosensor for detecting histamine, dendritic porphyrin-incorporated nanofibers, PCL-Por and PCL-Por(Zn), were fabricated by electrospinning. The resulting electrospun nanofibers exhibited a fully interconnected pore structure with narrow pore size distribution. EDX, ATR–FTIR and XPS results clearly revealed the existence and homogenous distribution of dendritic porphyrin on the surface of nanofibers. PCL-Por(Zn) nanofiber showed the fluorescence spectra change after the reaction with various histamine solutions. It could be seen that even in the lower histamine solution the fluorescence intensity reduced with increasing histamine concentration, in which histamine probably acted as a quencher. This is due to the complex formation between histamine and Zn of Den-Por(Zn) introduced into the PCL-Por(Zn) nanofiber via a coordination. Ó 2012 Elsevier Ltd. All rights reserved.

1. Introduction Histamine is regarded as one of the chemical mediators of allergy, gastric acid secretion and neurotransmission [1,2]. It is generally known as autacoid, and its physiological activities, such as extension of capillaries and contraction of smooth muscles, have been investigated [3]. Histamine is generally stored in mast cells and basophils, and it is released by a specific antigen and chemical stimulation [1,4]. Measurement of histamine in human urine is an important clinical parameter in various diseases, such as bronchial asthma, atopic dermatitis, acute allergic reactions and cancer [5]. Therefore, it is important for clinical and pathological investigations to establish a precise, sensitive and simple method for the measurement of histamine in human body fluids. Many methods can be used to detect histamine including capillary zone electrophoresis, gas chromatography (GC), high-performance liquid chromatography (HPLC), enzymatic assay, flow immunoassay and spectrofluorome⇑ Corresponding author. Tel.: +82 53 850 2512; fax: +82 53 850 3292. E-mail address: [email protected] (Y.-J. Kim). 0014-3057/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.eurpolymj.2012.09.011

try [6–11]. Some of these methods are in widespread use because of the low detection limit and high selectivity to histamine. However, complicated and multistep processes are required and are not suitable for simple and rapid detection of histamine. A method using fluorescent probes would be suitable for the real-time noninvasive monitoring of biomolecules because it would permit a change in the emission intensity to be detected on reaction with target molecules under mild conditions. Numerous fluorescent probes have been developed for the detection of biomolecules such as protein, DNA and metal ions in living cells [12–14]. However, so far a probe for histamine has been hardly reported. Porphyrins are important classes of conjugated organic molecules, which mimic the active site of many important enzymes such as hemoglobin and myoglobin [15]. Porphyrins are intensely colored biological molecules that change their absorbance wavelength maxima upon selective interaction with different reagents or under different condition [16,17]. Thus there has been growing interest in using porphyrins as active materials for analytical applications. In particular, metalloporphyrins have excellent electrocatalytic properties towards the detection of many biochemical

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analytes like nitric oxide and histamine [18,19]. However, most of them focus on disordered porphyrin dissolved in solution. The increment of surface area of the detector substrate is sure to increase the number of sensing sites available without increasing the amount of overall sample required [20,21]. Polymeric nonwoven membranes can be prepared by electrospinning, which have small pore size, high porosity and large surface-to-volume ratio. The interconnected porous nanofiber networks in electrospun membranes are useful for various applications such as gas separation, protein purification and wastewater treatment [22–24]. Among various applications, this porous structure is particularly ideal as a sensor substrate because it can provide a very large surface area for sensing and easy access of target molecules to the sensing sites [20]. With the great progress made in nanoscience and nanotechnology, interest has been increasing in exploring the assembly of porphyrin with various nanostructures including carbon nanofiber that has larger surface-to-volume ratio [16]. Electrospinning provides a mechanism to produce nanofibrous membranes from a variety of polymer materials including synthetic and natural polymers. Therefore, these nanofibrous membranes can be used as substrates for sensing various molecules such as volatile organic compounds, heavy metals and biomolecules. The aim of this study was to develop a new method for the detection of histamine with dendritic porphyrin-incorporated nanofibers, in which dendritic porphyrin used as an active material was synthesized according to our previous report (Fig. 1) [25]. Nanofibers were fabricated by electrospinning and their morphological change by the introduction of dendritic porphyrin was systematically examined. Furthermore, the fluorescence intensity change of dendritic porphyrin after the reaction with various concentration of histamine was investigated.

ride (MC) and 0.01 M phosphate buffered saline (PBS, pH 7.4) were purchased from Sigma–Aldrich Co. and used without further purification. Dendritic porphyrins (DenPor and Den-Por(Zn)) were synthesized according to the previous literature [25]. Other reagents were commercially available and were used as received.

2. Experimental

The morphology of PCL nanofibers containing dendritic porphyrin was observed by a field emission-scanning electronic microscope (FE-SEM, JSM-6335F, JEOL). Prior to SEM observation, all of the samples were coated with gold. The average diameter of nanofibers was determined by analyzing the SEM images with an image analyzing software

Polycaprolactone (PCL, Mw = 80,000), zinc chloride (ZnCl2), N,N-dimethylformamide (DMF), methylene chlo-

The electrospinning setup utilized in this study consisted of a syringe and needle (ID = 0.41 mm), a ground electrode, and a high voltage supply (Chungpa EMT Co., Korea). The needle was connected to the high voltage supply, which could generate positive DC voltages up to 40 kV. For the electrospinning of PCL nanofibers including dendritic porphyrin (Den-Por or Den-Por(Zn)), PCL was first dissolved in mixed solvent of MC/DMF at a ratio of 8:2 to obtain a concentration of 9 w/v% solution. Dendritic porphyrin with content of 3 wt.% based on the weight of PCL was added to PCL solution by dissolving it in a small amount of DMF. PCL solution containing dendritic porphyrin held in a 10 ml syringe was delivered into a needle spinneret by a syringe pump (KDS 100, KD Scientific Inc., USA) with a mass flow rate of 1.5 ml/h. The steel needle was connected to an electrode of a high voltage supply and a grounded stainless steel plate was placed at 15 cm distance from the needle tip to collect the nanofibers. The positive voltage applied to the polymer solutions was 17 kV. All experiments were carried out at room temperature and below 40% relative humidity. After the electrospinning, the nanofibrous membrane was carefully peeled off from the stainless steel plate and put into oven under 40 °C for 12 h. 2.3. Characterization of nanofibers

Fig. 1. Schematic diagram of synthetic routes of dendritic porphyrins [25].

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2.1. Materials

2.2. Electrospinning

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Fig. 2. SEM micrographs of (a) PCL, (b) PCL-Por and (c) PCL-Por(Zn) nanofibers.

Fig. 3. EDX N- and Zn-mapping micrographs of PCL-Por(Zn) nanofiber surface.

(Image-Pro Plus, Media Cybernetics Inc.). For the same samples of FE-SEM, the spectrum of energy dispersive X-ray spectroscopy (EDX) was applied to detect the dendritic porphyrin distribution profile on the nanofiber surface. ATR–FTIR spectra of the samples were obtained with an ALPHA spectrometer (Bruker Optics). X-ray photoelectron spectroscopy (XPS) measurements were carried out to analyze the surface chemical compositions of the samples using a Quantera SXM (ULVAC–PHI) equipped with monochromated Al-Ka X-ray source (1486 eV). Survey scan and high resolution C1s peaks were recorded at a take-off angle 45° relative to the sample surface. 2.4. Detection of histamine using nanofibers Histamine sensing capability of dendritic porphyrinincorporated nanofibers was assessed by depositing various concentrations of histamine solution (30 ll) on the surface of nanofibers (1  1 cm). These nanofibers were placed on the table for 20 min and then put into vacuum oven under 40 °C for 1 h. After drying, nanofibers were fixed to the steel chamber and the fluorescence emission spectra were measured with a Perkin–Elmer LS55 spectrometer at 25 °C. 3. Results and discussion 3.1. Morphology of electrospun nanofibers Electrospun nanofibers have recently received much attention as a sensor substrate because of high porosity

and large surface-to-volume ratio. These main advantages of the nanofiber substrates are that they can offer easy access of target molecules to the sensing sites. In this study, nanofibers were prepared by electrospinning from polycaprolactone (PCL) solutions containing 3 wt.% Den-Por (PCL-Por) or Den-Por(Zn) (PCL-Por(Zn)) based on the weight of PCL. The morphological structures of electrospun nanofibers are shown in Fig. 2. The resulting electrospun nanofibers exhibited a fully interconnected pore structure with narrow pore size distribution. The average diameter of electrospun nanofibers was hardly changed by the addition of dendritic porphyrins, which was 455 ± 42 nm for PCL nanofiber, 461 ± 25 nm for PCL-Por nanofiber and 446 ± 31 nm for PCL-Por(Zn) nanofiber. The solution viscosity is the most important parameter in electrospinning, which can affect the morphology of nanofibers, with the result that higher viscosity gives rise to the increase of fiber diameter [26]. However, the addition of dendritic porphyrins scarcely affected the variation of solution viscosity (51.0 cP), resulting that no change of fiber diameter was observed. The distribution of dendritic porphyrins on the electrospun nanofibers was measured with the EDX. Fig. 3 presents the EDX N- and Zn-mapping analysis of PCL-Por(Zn) nanofiber. The bright spots representing N or Zn element clearly reflected a homogenous distribution of dendritic porphyrin on the external surface of nanofiber. The incorporation of dendritic porphyrins into the nanofibers was confirmed by the investigation of color change and fluorescence response of the nanofibers under ambient light and UV irradiation. As expected, the addition of dendritic

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Fig. 5. ATR–FTIR spectra of (a) PCL, (b) PCL-Por and (c) PCL-Por(Zn) nanofibers.

porphyrin obviously caused the color change of PCL nanofiber to deep purple under ambient light (Fig. 4(a)). In addition, when excited under UV irradiation at 365 nm, fluorescence of dendritic porphyrin-incorporated nanofibers exhibited a color change from nonfluorescent to bright red (Fig. 4(b)). 3.2. Properties of nanofibers ATR–FTIR analysis was carried out for surface characterization of PCL, PCL-Por and PCL-Por(Zn) nanofibers in the range of 400–4000 cm–1. As shown in Fig. 5, the characteristic two peaks centered at 3440 and 1723 cm–1 were observed for the PCL nanofiber. The former one, due to the hydroxyl stretching vibration, was relatively weak, and the latter one was derived from the C=O stretching of ester group [27]. Furthermore, in the C–H stretching

region of FTIR spectrum, the higher intensity peak at 2942 cm–1 was assigned to the asymmetric and the lower intensity peak at 2861 cm–1 was assigned to the symmetric modes of CH2. In the case of dendritic porphyrin-incorporated nanofibers, PCL-Por and PCL-Por(Zn), the characteristic absorption band was observed at 1598 cm–1 associated with aromatic ring skeletal vibration of dendritic porphyrin. However, other characteristic bands of dendritic porphyrins were significantly overlapped with the absorption bands of PCL. The surface chemical composition of dendritic porphyrin-incorporated nanofibers was determined by XPS. Moreover, to obtain further insight into the chemical bonds present on the surface of the nanofibers, high resolution XPS C1s spectra can be deconvoluted by curve fitting technique. Fig. 6 shows the decomposed C1s peaks of nanofibers and the fraction of different carbon functional groups are given in Table 1. As shown in Fig. 6, the spectra from the nanofibers were resolved into three components; a peak at 285.0 eV corresponding to C–C bond, a peak at 286.5 eV assigned to C–O and C–N bonds, and a peak at 288.0 eV due to C=O bond [28,29]. The saturated C–C C1s peaks decreased from 67.2% to 63.7% by the introduction of dendritic porphyrins into the PCL nanofiber (Table 1). In addition, the intensity of C=O bond also slightly reduced from 12.3% to 10.4%. However, C1s peak ascribed to C–O and C–N functional groups increased from 20.5% to 25.8% with the use of dendritic porphyrins, which can be attributed to the increase of ether and pyrrole moieties on the surface of PCL-Por and PCL-Por(Zn) nanofibers by the incorporation of dendritic porphyrins. 3.3. Detection of histamine using nanofibers The detection of histamine is an important issue in clinical chemistry and drug discovery. A high level of histamine may cause a fatal allergic reaction [5]. The clinical significance of histamine arises from the fact that an

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Fig. 4. The color and fluorescence changes of the dendritic porphyrin-incorporated nanofibers under (a) ambient light and (b) UV irradiation at 365 nm. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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Fig. 6. XPS C1s spectra with peak deconvolution for (a) PCL, (b) PCL-Por and (c) PCL-Por(Zn) nanofibers. Table 1 Fraction of carbon function groups from high resolution C1s XPS peaks.

PCL PCL-Por PCL-Por(Zn)

C–C (%) (285.0 eV)

C–O/C–N (%) (286.5 eV)

C=O (%) (288.0 eV)

67.2 65.6 63.7

20.5 24.0 25.8

12.3 10.4 10.5

increased level of histamine in blood is associated with pathological conditions such as gastric disorders, mastocytosis and chronic myelogenous leukemia [30]. Thus sensitive detection of histamine is helpful for clinical and pathological investigations. To develop new fluorescent probe for the detection of histamine, we synthesized the

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dendritic porphyrins without or with Zn (Den-Por or Den-Por(Zn)). The spectrophotometric property of these dendritic porphyrins was first examined with a Hitachi U-2900 spectrometer. The UV absorption spectra of dendritic porphyrins are due to the electronic transitions from the ground state (S0) to the lowest singlet excited states S1 (Q state) and S2 (Soret state) [31]. As shown in Fig. 7, Den-Por exhibited strong absorption at 408 nm with four minor Q bands at 501, 535, 574 and 629 nm, respectively, in DMF. The Soret band at 409 nm can be attributed to the p–p⁄ transition from the ground state S0 to the second excited singlet state S2. The four Q bands are due to the p–p⁄ transitions from the ground state S0 to the different vibration levels of the first excited singlet state S1 [32]. In the case of Den-Por(Zn), the absorption bands appeared at 415 nm (Soret band), 544 and 581 nm (Q bands). These spectral patterns are similar to those of general Zn-containing

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Fig. 7. UV absorption spectra of Den-Por and Den-Por(Zn) (50 lM) in DMF.

1993

Fig. 8. Fluorescence spectra of the dendritic porphyrin-incorporated nanofibers, (a) PCL-Por and (b) PCL-Por(Zn), after the reaction with various histamine solutions.

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Fig. 9. Fluorescence spectra of Den-Por(Zn) (50 lM) in a mixture of DMF and 0.01 M PBS (pH 7.4) with various concentrations of histamine.

porphyrins [31]. The Soret band was shifted to the red region by 7 nm compared to the Den-Por spectrum. Moreover, the decrease in the number of Q bands and the shift of absorption peaks to the red region by 7–9 nm were observed. This may be due to the result of the increased molecular symmetry of Den-Por(Zn) by the coordination of tetrapyrrolic center of porphyrin with Zn ion [33,34]. The fluorescence emission spectra (kex = 420 nm) of Den-Por and Den-Por(Zn) could be obtained from the measurement of dendritic porphyrin-incoporated PCL nanofibers, PCL-Por and PCL-Por(Zn) (Fig. 8). Furthermore, the fluorescence response of dendritic porphyrins to various concentrations of histamine was assessed by the deposition of histamine solutions on the nanofibers. Due to rapid vibronic relaxation from S2 to S1, only emissions at 635 and 700 nm corresponding to the (0,0) and (0,1) transitions in PCL-Por nanofiber were observed and there were no emission peaks near the Soret band [25]. The fluorescence spectra change of PCL-Por nanofiber was not detected even after reaction with the histamine solutions, meaning that the complex formation or intermolecular interaction between Den-Por and histamine on the nanofiber was hardly occurred. Den-Por(Zn)-bearing PCL-Por(Zn) nanofiber exhibited similar emission pattern with that of PCL-Por, but a large blue shift compared to the spectra of PCL-Por was detected, in which the (0,0) and (0,1) emissions were measured at 590 and 640 nm. This spectral change is a general phenomenon of Zn-containing porphyrin and maybe caused by the effect of the axis coordination of pyrrole nitrogen with Zn ions as described above [31,34]. This indicates that the coordination between porphyrin and multivalent metal ions can give rise to the reduction of bandgap and blue shift of emission [35]. In addition, contrary to PCL-Por nanofiber, PCL-Por(Zn) nanofiber showed the fluorescence spectra change with the deposition of various concentrations of histamine solutions. The fluorescence intensity at 590 and 640 nm was suppressed with increasing the concentration of histamine as shown in Fig. 8(b). The

Fig. 10. Fluorescence spectra of various concentrations of histamine in distilled water (a) without and (b) with Zn2+ ions (100 lM).

fluorescence intensity of nanofiber was also changed even after the reaction with low levels of histamine. It was previously found that histamine could be combined with various metal ions such as Cu2+, Ni2+ and Zn2+ to form a histamine–metal ion complex [36]. Therefore, in this study, imidazole nitrogen of histamine was bound to the metal center of Den-Por(Zn) via a coordination. In conclusion histamine probably acted as a quencher and the fluorescence intensity of PCL-Por(Zn) nanofiber reduced. This observation indicates that PCL-Por(Zn) nanofiber is a potential candidate as fluorescent chemosensor for the detection of histamine. To prove the effect of histamine on the fluorescence quenching of Den-Por(Zn), the fluorescence spectra change of Den-Por(Zn) (50 lM) in a mixture of DMF and 0.01 M PBS (pH 7.4) with various concentrations of histamine was precisely assessed. As shown in Fig. 9, the fluorescence intensity at 590 and 640 nm decreased with the increment of histamine concentration. This is in very good agreement with the result in the solid state of Den-Por(Zn) using PCLPor(Zn) nanofiber. Furthermore, the complex formation of histamine with Zn2+ ions was investigated for the explanation concerning the coordination of histamine in the metal center of Den-Por(Zn). Histamine without Zn2+ ions revealed the emission maxima at 485 and 488 nm (kex = 490 nm) and their fluorescence intensity increased

with the increase of histamine concentration (Fig. 10(a)). In the presence of Zn2+ ions, the fluorescence intensity of histamine also rose with concentration as shown in Fig. 10(b). However, the emission maxima were slightly red-shifted by the addition of Zn2+ ions and lower fluorescence intensity was detected than that for the same concentration of Zn2+ ion-free histamine solutions, suggesting that histamine was bound to Zn2+ ions and quenched. This result can clearly explain the complex formation between histamine and Zn of Den-Por(Zn) via a coordination.

[9]

[10]

[11]

[12]

4. Conclusions [13]

A precise assessment of histamine in vivo and in vitro is important issues in the field of biological science. Recently, many researchers have endeavored to develop novel fluorescent chemosensor for detecting various biomolecules. In the present study, dendritic porphyrin-incorporated PCL nanofibers have been developed by the electrospinning technique for potential use in sensor application. The resulting nanofibers showed a porous morphology formed by interlaying of the fibers. PCL-Por(Zn) nanofiber showed the fluorescence spectra change after the reaction with various histamine solutions. The fluorescence intensity was suppressed with increasing the histamine concentration and the low levels of histamine could be measured. This detection is based on the fluorescence quenching of Den-Por(Zn) in PCL-Por(Zn) nanofiber with histamine. It is concluded that this new method for detecting histamine may be helpful to develop a simple fluorescent chemosensor usable in the solid state. Acknowledgements

[14]

[15]

[16]

[17] [18]

[19]

[20]

[21]

[22]

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0002839).

[23]

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