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Molecular filters based on cyclodextrin functionalized electrospun fibers
Journal of Membrane Science 332 (2009) 129–137
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Molecular filters based on cyclodextrin functionalized electrospun fibers Tamer Uyar a,∗ , Rasmus Havelund a , Yusuf Nur c , Jale Hacaloglu c , Flemming Besenbacher a,b , Peter Kingshott a,∗∗ a
Interdisciplinary Nanoscience Center (iNANO), Aarhus University, DK-8000 Aarhus C, Denmark Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark c Department of Chemistry, Middle East Technical University, Ankara 06530, Turkey b
a r t i c l e
i n f o
Article history: Received 26 November 2008 Received in revised form 26 January 2009 Accepted 27 January 2009 Available online 6 February 2009 Keywords: Cyclodextrin Electrospinning Nanofiber Polystyrene Nanofilter
a b s t r a c t Beta-cyclodextrin (-CD) was successfully incorporated into polystyrene (PS) fibers by electrospinning technique. The subsequent fibrous membranes show potential for efficient removal of organic compound (e.g.: phenolphthalein) from solution by the formation of inclusion complexes with the -CD molecules. Since the filtration efficiency of the fiber membranes is highly dependent on the presence and distribution of -CD at the surface of the individual fibers, highly sensitive and specific surface spectroscopic analyses were carried out. X-ray photoelectron spectroscopy (XPS) was used to quantify the amount of -CD on the surface of PS fibers. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) showed the presence of both -CD molecular and fragment ions on the fiber surfaces, and high resolution chemical imaging demonstrated even distribution of -CD on the surface of the individual fibers. Phenolphthalein (PhP) was used as a model compound in the filtration studies, and a strong dependence was observed between -CD content of the PS/CD fibers and efficiency of trapping PhP. These results demonstrate the potential of using PS/CD fibrous membranes to filter organic molecules in purification/separation processes. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Electrospinning is a technique for producing fibers of micrometer and nanometer diameters by creating a continuous filament by exposing a polymer solution or polymer melt to very high electrical fields. Due to its versatility and cost effectiveness electrospinning has gained wide use recently to produce multi-functional nanofibers from different materials, which include polymers, polymer blends, ceramics, sol–gels and composite solutions with nanoparticles [1,2]. Nanofibers/nanowebs produced by electrospinning have several remarkable characteristics such as very large surface area to volume ratio, pore sizes in nanorange, unique physical and mechanical performance along with the design flexibility for chemical/physical surface functionalization. It has been shown that the outstanding properties and multi-functionality of such nanofibers/nanowebs make them favorable candidates for many applications in biotechnology, textiles, membranes/filters, composites, sensors, etc. [1–5].
∗ Corresponding author. Present address: UNAM-Institute of Materials Science & Nanotechnology, Bilkent University, Ankara 06800, Turkey. Tel.: +45 8942 3553; fax: +45 8942 3690. ∗∗ Corresponding author. Tel.: +90 312 2903571; fax: +90 312 2664365. E-mail addresses: [email protected], [email protected], [email protected] (T. Uyar), [email protected] (P. Kingshott). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.01.047
We report here a new method for taking advantage of the high surface-mass ratio of fibers to form molecular filters and/or nanofilters, with the added advantage of incorporating cyclodextrins (CDs) as functional additives that can trap molecules and function as highly efficient filtration systems. CDs are cyclic oligosaccharides consisting of 1,4-linked glucopyranoside units having either six, seven, or eight glucose units arranged in a cyclic structure, named as ␣-, - and ␥-cyclodextrins, respectively (Fig. 1). The hydrophobic cavity of CDs allows them to form noncovalent host–guest inclusion complexes (CD-ICs) with various molecules [6,7]. The formation of CD-IC and its stability depend on many factors such as the size/shape fit and binding forces between the host CD molecules and the guest molecules, and the chemical surroundings [8]. Since the physical and chemical properties of incorporated guest compounds can be tailored by complexation with CDs, the CDs are used in a variety of application areas, such as pharmaceuticals, foods, cosmetics, home/personal care, textiles, etc. [6,7]. Moreover, cyclodextrins and cyclodextrin functionalized materials are also used in filters and membranes for separation/purification/filtration purposes [9–13]. The electrospun nanofibrous mats can both physically filter tiny particles and remove industrial waste [5,14,15]. The combination of cyclodextrins and the electrospun nanofibers can potentially increase the efficiency of filters by facilitating complex formation with organic compounds and the very high surface area of the nanofibers. There are only a limited number of studies
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Fig. 1. (a) Chemical structure of -CD and (b) approximate dimensions of ␣-, -, ␥-CDs.
in the literature that report incorporation of CDs into electrospun fibers for various purposes [16–23]. For instance, -CD have been used to crosslink poly(acrylic acid) nanofibers in order to produce water-insoluble polyelectrolyte nanowebs [16]. The composite electrospun poly(N-vinylpyrrolidone) (PVP) nanofibers were obtained with -CD [17] and in a later study PVP nanofibers containing gold nanoparticles have been produced in which -CD was used as a stabilizing and reducing reagent [18]. Furthermore, a catalyst for the detoxification of nerve agents has been synthesized from -CD and o-iodosobenzoic acid. This modified -CD was incorporated into polyvinyl chloride (PVC) nanofibers in order to develop functional nanofibrous membranes that can provide protection from chemical warfare stimulant [19]. Poly(methyl methacrylate) (PMMA) nanofibers have been electrospun with phenylcarbomylated -CD in order to try to capture organic molecules for waste treatment [20]. In our recent studies we have produced poly(ethylene oxide) (PEO) nanofibers containing cyclodextrin/poly(ethylene glycol) (PEG) inclusion complex [21]. We have also successfully produced electrospun PMMA [22], PEO [23] and polystyrene (PS) nanofibers [24] functionalized with cyclodextrins. Furthermore, we incorporated CD–menthol inclusion complex in electrospun PS and PMMA nanofibers for the purpose to produce functional nanofibers that contain fragrances/flavors with high temperature stability [25,26]. In the present study we produced cyclodextrin functionalized electrospun polystyrene nanofibers (PS/CD) with the goal to develop functional nanowebs and we have shown that these CD functionalized nanowebs may have the potential to be used as molecular filters and/or nanofilters for filtration/purification/separation purposes. Polystyrene was chosen as a fiber matrix since PS does not form inclusion complexes with -CD as the cavity of -CD is too narrow to encapsulate atactic PS chains [27,28]. Thus, the cavity of -CD molecules will be empty and able to capture organic molecules. We studied the filtration process by using phenolphthalein (PhP) as a model organic molecule since it forms inclusion complexes with -CD and can be readily monitored by adsorption measurements. We investigated the surface composition and homogeneity of individual PS/CD fibers using highly surface sensitive spectroscopic techniques, X-ray photoelectron spectroscopy (XPS) and imaging time-of-flight sec-
ondary ion mass spectrometry (ToF-SIMS). These two techniques can give information on the surface compositions and spatial distribution of molecules on the surface of individual fibers. Such knowledge is highly desirable for understanding and optimizing the properties of modified fibrous materials. Finally, evidence of inclusion complexation of phenolphthalein by the CD molecules located on the fibers was confirmed both by UV–vis spectroscopy and direct pyrolysis mass spectrometry (DP-MS) studies. 2. Experimental 2.1. Materials Amorphous polystyrene (Mw ∼ 280,000), N,N-dimethylformamide (DMF) (99%), phenolphthalein (ACS reagent) and ethanol (absolute, HPLC grade, ≥99.8%) were purchased from Sigma–Aldrich. Beta-cyclodextrin (-CD) was a gift from Wacker Chemie AG (Germany). The materials were used without any purification. 2.2. Electrospinning The homogeneous clear solutions were prepared by dissolving PS and -CD in DMF at room temperature. The polymer concentration was varied from 15% to 25% (w/v) and the -CD content was varied from 10% to 50% (w/w) with respect to polymer. The polymer solutions were placed in a 1 mL syringe fitted with a metallic needle of 0.4 mm inner diameter. The syringe is fixed horizontally on the syringe pump (Model: KDS 101, KD Scientific), and a electrode of high voltage power supply (Spellman High Voltage Electronics Corporation, MP Series) was clamped to the metal needle tip. The flow rate of polymer solution was 1 mL/h and the applied voltage was 15 kV. The tip-to-collector distance was set to 10 cm and a grounded stationary rectangular metal collector (15 cm × 20 cm) covered by a piece of clean aluminum foil was used for the fiber deposition. The whole electrospinning apparatus was enclosed in glass box, and the electrospinning was carried out in a horizontal position at room temperature. The fibers collected on aluminum foil were dried at 40 ◦ C under vacuum oven for 24 h to remove the residual solvent.
Table 1 The properties of PS and PS–CD solutions and the resulting electrospun PS and PS/CD fibers. Materials
% PSa (w/v)
% CDb (w/w)
Fiber diameterc (nm)
Fiber morphology
Viscosity (cP)
PS10 PS15 PS20 PS25 PS20/CD10 PS15/CD25 PS10/CD50
10 15 20 25 20 15 10
– – – – 10 25 50
– – – 1959 ± 1610 ± 1214 ± 1161 ±
Beaded nanofibers Beaded nanofibers Beaded nanofibers Bead-free microfibers Bead-free microfibers Bead-free microfibers Bead-free microfibers
22.9 59.9 138.3 240.4 146.9 60.6 21.1
a b c
With respect to solvent (DMF). With respect to polymer (PS). Only bead-free fibers were reported.
162 186 279 271
± ± ± ± ± ± ±
0.1 0.5 0.7 1.5 0.7 0.2 0.3
Conductivity (S/cm) 2.5 1.35 1.1 1.1 2.4 3.8 5.3
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Fig. 2. SEM images of (a) PS25, (b) PS20/CD10, (c) PS15/CD25, and (d) PS10/CD50.
2.3. Measurements and characterization The viscosity of the polymer solutions was measured by using Brookfield DV-III Ultra Rheometer equipped with cone/plate accessory using the spindle type CPE-41. The viscosity measurements were repeated three times or more to ensure consistent viscosity readings. The conductivity of the solutions was measured with a Multiparameter meter InoLab® Multi 720 (WTW) at room temperature. The fiber morphology was examined by high resolution scanning electron microscopy (SEM) (FEI, Nova 600 NanoSEM) at 10 kV. The aluminum foil with the nanofibers was directly put into the SEM chamber without any metal sputtering or coating. The average fiber diameter was calculated from SEM images and around 50 fibers were measured. X-ray photoelectron spectroscopy was performed using a Kratos Axis UltraDLD instrument equipped with a monochromated Al K␣ X-ray source (h = 1486.6 eV) operating at 10 kV and 15 mA (150 W). The nanofiber samples that were analyzed were cut from the Al foil containing a thick layer of fibers. A hybrid lens mode was employed during analysis (electrostatic and magnetic), with an analysis area of approximately 300 m × 700 m. For each sample, a take-off angle (TOA) of 0◦ (with respect to the sample surface) was used allowing a maximum probe depth (10 nm). Wide energy survey scans (WESS) were obtained over the range 0–1400 eV binding energy (BE) at a pass energy of 160 eV, and used to determine the surface elemental composition. High resolution spectra were recorded for C 1s and O 1s at a detector pass energy of 20 eV. The Kratos charge neutralizer system was used on all samples with a filament current of between 1.8 and 2.1 A and a charge balance of 3.6 V. Sample charging effects on the measured BE positions were
corrected by setting the lowest BE component of the C 1s spectral envelope to 285.0 eV, corresponding to the C–C/C–H species. Deconvolution of the high-resolution spectral regions was performed by subtraction of a linear background and application of a mixed Gaussian–Lorentzian synthetic peak. Full width at halfmaximum (FWHM) values for each component were often set to values that gave the best fits and made the most physical sense in relation to the resolving power of the instrument and experimental parameters used. ToF-SIMS analysis was performed using an ION-TOF TOF.SIMS 5 instrument equipped with a Bi primary ion cluster source operating at 25 kV. In most cases Bi3 + primary ions were used with a target current of 0.3 pA. High-resolution mass spectra (M/M > 4000 at m/z = 27) were acquired using the high current bunched mode. High mass resolution spectra were obtained by the use of bunched primary ions. The analysis area was 500 m × 500 m. Only positive secondary ions spectra were acquired for in-depth analysis. Mass calibration of the spectra was based on CH3 + , C2 H3 + , C2 H5 + , and C3 H5 + ions. Chemical imaging was performed with the burst alignment mode which offers a high spatial resolution but low mass resolution. Due to the low mass resolution, chemical imaging was based only on peaks unambiguously identified from high mass resolution spectra over an area of 75 m × 75 m. In all cases an electron flood gun was for charge compensation and the primary ions dose was kept below 1012 ions/cm2 to stay within the static SIMS regime. The filtration performance of the PS and PS/CD fibers was tested using phenolphthalein as a model organic molecule. Uptake was determined by measuring the reductions in absorbance or depletion from solution using UV–vis spectrophotometry (Helios-) in the wavelength range of 400–700 nm. The weight of fiber mass
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Fig. 3. Deconvoluted XPS C 1s spectra for (a) the PS fibers, (b) pure -CD, (c) PS20/CD10 fibers, (d) PS15/CD25 fibers, and (e) PS10/CD50 fibers.
and the amount of PhP used were calculated under the assumption that 10% CD (w/w) is present on the fiber surface and thus available for complexation and 1:1 complexation between CD and PhP molecules occurs. A 4 × 10−4 M of PhP solution was prepared in absolute ethanol. The pH of the solution was adjusted to basic (above pH 11) by adding drops of pH buffer solution. The pH of the solutions was measured with a pH meter (pH meter 1500, EUTECH Instruments) before and after the UV–vis experiments; the pH of the solutions was unchanged during the course of the UV–vis experiments. 16 mg of PS and PS/CD fibers were placed separately in the bottom of the UV–vis cuvettes filled with PhP solution. The cuvettes were covered tightly with a Teflon lid to prevent the evaporation of ethanol and allowed to stand undisturbed 3 days. The absorbance spectra of the PhP solutions were recorded initially (time = 0, right after adding the fibers) and every 24 h. After 3 days the fibrous mats were removed, rinsed with ethanol and water to remove physically adsorbed PhP molecules, and analyzed by direct pyrolysis mass spectrometry. The DP-MS system consists of Waters Quattro MicroGC tandem MS with an EI ion source and a mass range of 10–1500 Da coupled with a direct insertion probe (Tmax = 650 ◦ C). 0.01 mg of fiber was pyrolyzed in flared quartz sample vials. The temperature was increased at a rate of 10 ◦ C/min and the scan rate was 1 scans/s, with simultaneous mass spectrometric analysis of the pyrolytic fragments. 3. Results and discussion
varied from 10% (w/w) to 50% (w/w) with respect to PS. The characteristics of PS and PS/CD solutions and the resulting electrospun fibers are summarized in Table 1. The scanning electron microscopy images of electrospun PS fibers and PS/CD fibers containing 10%, 25% and 50% (w/w) -CD are shown in Fig. 2. For PS solutions, beaded fiber structures were obtained at lower polymer concentrations (10–20%, w/v) but an increase in the polymer concentration to 25% (w/v) yielded bead-free fibers (Fig. 2a), which indicates that a high viscosity is required to obtain uniform PS fibers. This finding is consistent with previous findings in the literature where bead-free PS fibers were obtained only at the high concentration/viscosity range yielding microfibers [29,30]. On the contrary, the PS/CD systems yielded bead-free uniform fibers even at low concentrations of PS (10–20%, w/v) depending on the CD content. The positive effects of CD on electrospinning of bead-free PS fibers from low concentrations were discussed in detail in a previous publication [24]. In brief, 20% PS/10% -CD, 15% PS/25% -CD and 10% PS/50% -CD combinations yielded bead-free fibers. The viscosity of the PS/CD solutions was almost the same as the pure PS solutions indicating the negligible effect of -CD. However, the addition of the -CD increased the conductivity of polymer solutions, and resulted in large differences in fiber morphology of fibers that were bead-free. It is also worth noting that the average diameter of the PS/CD fibers was narrower since low PS concentrations were used. Although the average fiber diameters for the PS and PS/CD electrospun systems are in micron range, nanofibers from these systems can be easily achieved by increasing the conductivity of the solution, that is, by adding small amount of salt to the polymer solutions [30].
3.1. SEM characterization 3.2. XPS and ToF-SIMS characterization The optimization of the electrospinning conditions for producing uniform cyclodextrin functionalized PS fibers (PS/CD) and the morphological and bulk chemical characterization of the PS/CD fibers have been reported previously [24]. The concentration of the PS solutions was varied from 10% (w/v) to 25% (w/v), and -CD was
Of particular interest are the surface properties of the PS/CD since they determine the functionality of filter, and during electrospinning it is quite possible that some CD molecules may phase separate from the PS matrix and reside on the surface of the fibers.
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Table 2 Atomic concentrations generated from XPS wide energy survey scans. Sample
C (%)
O (%)
PS -CD PS20/CD10 PS15/CD25 PS10/CD50
100 58.7 98.8 98.0 94.3
0 41.3 1.2 2.0 5.7
XPS and ToF-SIMS analyses were performed on the different PS/CD fibers in order to verify both the amount of CD present on the surfaces (XPS), and the lateral distribution of CD molecules present on individual PS/CD fibers (ToF-SIMS). This is important if the CD molecules are to be used to capture molecules from solvents that do not swell the PS matrix, since they will only be exposed to the surface. Table 2 shows XPS atomic concentrations generated from wide energy survey scans for PS fibers, pure -CD, and PS/CD fibers. The data clearly demonstrate that the surface of the PS/CD fibers is mainly PS with small amounts of CD present as indicated by the low oxygen content compared to pure CD. An increase in oxygen content is observed for increasing concentration of CD in the PS solution. The measured probe depth under the experimental conditions chosen is ∼10 nm and the spot size is 700–300 m, thus an average surface composition is provided. The chemical functionality of the electrospun fibers was evaluated from the core level C 1s spectra as shown in Fig. 3 PS fibers, -CD and three different PS/CD fibers. For the pure PS fibers the peaks assigned as follows [31]: C C of the benzene ring of PS at 284.6 eV (peak C), the aliphatic carbon atoms of PS at 285.0 (peak B) and the –* shake-up satellites (peak A) around 291.5 eV. The pure -CD spectrum (Fig. 3b) shows three peaks that are assigned to C–C/C–H (small hydrocarbon contamination) (peak B), C–O–C/C–OH at 286.7 eV (peak D) and O–C–O/C O at 288.1 eV (peak E). The C 1s spectra for PS/CD fibers (Fig. 3c–e) are a composition of peaks from PS and CD, with PS peaks as the major components. A peak D at 286.4–286.8 eV is assigned to the C–O–C/C–OH species of -CD [32]. This peak component becomes more prominent with increasing -CD concentration, which is consistent with the elemental composition. The O–C–O/C O component in the pure -CD (peak E) is only visible for PS10/CD50. Based on the data in Table 2 it is estimated that about 3%, 5%, and 13% of the probed volume consists of -CD molecules for the PS20/CD10, PS15/CD25, and PS10/CD50 samples, respectively. The surface CD content for all three PS/CD samples is substantially lower than the CD content of the solutions they were prepared from, indicating that most the -CD molecules are buried in the bulk of the fibers. Thus, optimization of the experimental parameters to obtain high CD content on the fiber surfaces is desirable to improve the filtration efficiency of these fibrous membranes. With a depth resolution of 1–2 nm, ToF-SIMS was used to obtain molecular information of the PS/CD fiber surfaces, and assess the uniformity of the -CD distribution on individual PS/CD fibers surface. Initially spectra were acquired in high mass resolution mode to verify the presence of -CD at the surface of the samples through the identification of specific fragment ions. The positive ion spectrum for PS15/CD25 is shown in Fig. 4, and the assignments for selected ions are shown in Table 3 with accurate mass determination. The same peaks are observed in the spectra for the two other PS/CD fiber samples, thus making the assignment valid for all samples. The spectrum of pure PS (not shown) shows a specific pattern of peaks which can all be assigned to Cx Hy + ions with the most intense being the tropylium ion, C7 H7 + at m/z = 91. These peaks also appear in the PS/CD spectra. Both CD and PS can fragment into Cx Hy + ions
Fig. 4. Representative positive ion ToF-SIMS spectrum recorded from the surface of PS15/CD25 fibers. Major peaks assignments are shown in Table 3.
when x ≤ 5 and, thus, assignments of peaks to either PS or CD must be carried out carefully in this range. At higher mass, a number of peaks unambiguously identify -CD (see Table 3). The presence of -CD is most characteristically indicated by a peak at m/z = 1157, which is assigned to the single charged Na-adduct ion of the intact -CD molecule. Peaks at masses m/z = 163 and 325 correspond to the fragment ions of -CD assigned to one and two glucopyranoside units, respectively. A number of peaks in the lower mass range can be assigned to oxygen containing fragment ions. The high mass resolution allows for separation of these ions from the Cx Hy + ions originating from polystyrene. Table 3 Peak assignments derived from the positive ion TOF-SIMS spectra of PS–CD. Peak
Ion
Theoretical mass
Measured mass
77 91 103 117 193 69 73 81 85 87 97 101 163 325 1157
C6 H5 + C7 H7 + C8 H7 + C9 H9 + C15 H13 + C4 H5 O+ C3 H5 O2 + C5 H5 O+ C4 H5 O2 + C4 H7 O2 + C5 H5 O2 + C4 H5 O3 + C6 H11 O5 + C12 H21 O10 + C42 H70 O35 Na+
77.0391 91.0548 103.0548 117.0704 193.1017 69.0340 73.0290 81.0340 85.0290 87.0446 97.0290 101.0239 163.0606 325.1135 1157.3595
77.0412 91.0571 103.0504 117.0713 193.1025 69.0349 73.0356 81.0385 85.0345 87.0518 97.0284 101.0281 163.0749 325.1657 1157.3710
The peaks are used for chemical imaging shown in Fig. 5.
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Fig. 5. ToF-SIMS chemical images of fibers taken using the burst alignment mode. (a) Total ion image of PS15/CD25, (b) PS fragment ion image of PS15/CD25, and (c) CD fragment ion image of PS15/CD25. The distribution of CD on the surface of different PS/CD fibers shown by overlays of CD (green) on PS (red) fragment ions image, respectively (software color). (d) PS20/CD10, (e) PS15/CD25, and (f) PS10/CD50. Image area 75 m × 75 m. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
The fragment ions shown in Table 3 originate from either -CD or PS, and all appear at unique m/z values. These peaks are used to derive 2D chemical maps showing the distribution of either PS or CD molecules on the fiber surfaces. In this case the burst alignment mode of the ToF-SIMS instrument is used obtain high spatial resolution images, and to improve greater contrast either PS or -CD fragment ions by summing the intensity of the respective peaks. Fig. 5a–c shows secondary ion images of PS15/CD25 fibers over a 75 m × 75 m scan area. The respective images for PS and -CD are combined to form a single overlay image showing the distribution of CD on the surface of the fibers (Fig. 5e). Similar overlay images for the PS20/CD10 (Fig. 5d) and PS10/CD50 (Fig. 5f) are also shown. Clearly the individual fibers can be imaged by the contrast in the chemical composition between the PS and CD. The dominant features in the spectra are assigned to PS fragments indicating that PS is the major component on the surface, supporting the XPS data. However, all of the PS/CD samples contain regions on the fibers that show CD molecules. Some of these regions (green parts of Fig. 5) indicate aggregation of CD at the surface of the fibers. There are also some overlapping regions on the individual fibers, showing the presence of both PS and CD molecules and indicative of partial mixing between the two species. The localized distribution of
Table 4 The cumulative % decrease of PhP absorbance over time at 562 nm. Materials
PS25 PS20/CD10 PS15/CD25 PS10/CD50
Time (h) 0
24
48
72
0 0 0 0
11.64 10.41 28.33 35.2
12.30 24.25 42.41 53.64
11.37 33.29 52.88 63.25
CD at the surface of the fibers is seen in all PS–CD samples. The relative amount of CD within the larger CD clusters and the actual size of the clusters increases with increasing CD concentration. The observed distribution behavior will play a large role in determining the efficiency of the fibers as filters, and experiments are underway to optimize the electrospinning process. The improvement of the CD content on the fiber surfaces and the uniform dispersion of the CD may be achieved by varying the spinning conditions and parameters such as electrospinning at high temperature and/or using different solvent/co-solvent systems. An alternative approach might be to use chemically modified CD derivatives which tend to phasesegregate to the fiber surface during the solvent evaporation under the applied electrospinning process. 3.3. Uptake of phenolphthalein The trapping ability of -CD functionalized PS fibers was tested using phenolphthalein as a model system. PhP has often been used as a standard to test for CD inclusion complexation due to its high affinity for the CD cavity [33,34]. The PS and PS/CD fibers were immersed into a PhP solution and the change in absorbance of PhP was recorded as a function of time by UV–vis spectrometry (Fig. 6). It was observed that the absorbance of PhP solution decreased significantly over time in the presence of PS/CD fibers and pink solution become colorless, due to the removal of PhP from solution by -CD. Table 4 summarizes the cumulative % decrease of PhP absorbance over time. The PS/CD fibers function as a molecular filter through complexation of the PhP molecules with the -CD molecules on the surface of the PS fibers. For PS fibers, no color change was observed and only a small decrease (∼12%) in PhP absorbance was observed after the first day and stayed more or less constant afterwards. The small decrease in PhP concentration is most likely due to adsorption onto the surface of the fibers. The slight absorbance increase in
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Fig. 6. UV–vis spectra of phenolphthalein (PhP) solution as a function of time after dipping the webs of (a) PS25, (b) PS20/CD10, (c) PS15/CD25, and (d) PS10/CD50.
the third day is possible due to desorption of some PhP molecules from the fiber surface. On the other hand, for the PS/CD fibers, the decrease in PhP absorption was significant after the first day (in the range of ∼10–35% depending on the CD content), with continual decreases reaching ∼35–65% after 3 days. The decrease for PhP was about constant after 6 days (∼35%) for PS/CD10 fibers indicating that the saturation point was reached after approximately 3 days. It was observed that the higher the content of CD on the fibers, the more rapidly the PhP is removed from the solution (Table 4). These findings are very promising and show the potential application for cyclodextrin functionalized electrospun fibers to be used in filters for the removal of organic molecules. Ongoing studies are investigating the ability of these CD functionalized PS fibers at capturing organic molecules (e.g.: styrene, aniline and toluene) from the solutions. CD molecules have the ability to trap polluting substances (organic waste, heavy metals, radioactive wastes, etc.) from the environment [9–13] where the complexation is limited by the size of the waste molecules and CD cavity. We expect that such cyclodextrin functionalized electrospun nanofilters will be very applicable for waste treatments as long as the CD cavity can form complexation with the target molecules.
detection of a peak, but also the variation of its intensity as a function of temperature, i.e., evolution profile, is important. The trends in the evolution profiles can be used to determine the source of the product, or the mechanism of thermal degradation.
3.4. Direct pyrolysis mass spectrometry In order to confirm the complexation of -CD with phenolphthalein molecules, direct pyrolysis mass spectrometry analysis was performed on the PS/CD fibers after exposed to PhP solution. For a multi component system, DP-MS allows separation of components as a function of their volatilities and/or thermal stabilities. Once included in the host CD cavities, the thermal evaporation/decomposition of the guest molecules shifts to higher temperatures due to the strong interaction with the CD cavity, thus, DP-MS technique is a useful technique to characterize the CD host–guest inclusion complexes [35]. In general, DP-MS facilitates analyses of volatility, thermal stability and thermal degradation products of materials which can be used for investigation of chemical structures [36,37]. In pyrolysis MS analysis, not only the
Fig. 7. DP-MS evolution profiles of PS-based product; monomer (m/z = 104 Da), phenolphthalein (m/z = 318 Da) and CD-based product; C2 H4 O2 (m/z = 60 Da) detected during the pyrolysis of (a) pure samples (PS fibers, phenolphthalein and b-CD), (b) PS25, (c) PS20/CD10, (d) PS15/CD25, and (e) PS10/CD50. (Note: The fibers were analyzed after the exposure to PhP solution at the end of the UV–vis experiments.)
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PS and PS/CD fibers exposed to phenolphthalein solution were analyzed by DP-MS to investigate the presence of phenolphthalein and its thermal stability in the samples. DP-MS evolution profiles of PS fiber, phenolphthalein, -CD and PS/CD fibers are shown in Fig. 7. The evolution profiles of some diagnostic products of polystyrene, phenolphthalein and -CD namely; the styrene monomer (m/z = 104 Da), phenolphthalein (molecular peak, m/z = 318 Da) and C2 H4 O2 (m/z = 60 Da) respectively, detected during the pyrolysis are shown. The evolution of polystyrene and CD-based products generated during the pyrolysis of PS and PS/CD fibers exposed to phenolphthalein solution showed almost identical trends with the corresponding profiles detected during the pyrolysis of pure PS and CD. The pyrolysis of pure phenolphthalein started just above 170 ◦ C, maximizing around 257 ◦ C and completed around 280 ◦ C (Fig. 7a). In the case of pure PS fibers (Fig. 7b), the phenolphthalein was hardly detected, confirming that only a small amount of was adsorbed on the surface and washed out during the rinsing process. This correlates well with UV–vis absorbance results as discussed previously. On the other hand, all the evolution profiles of PS/CD fibers showed the presence of phenolphthalein. Moreover, the evolution of phenolphthalein shifted to higher temperatures during the pyrolysis of these samples (Fig. 7c–e); phenolphthalein evolution was maximized at 300 ◦ C for PS20/CD10 and PS10/CD50 samples, and at 350 ◦ C for PS15/CD25 sample. For the PS10/CD50 sample, a second maximum and, for the PS20/CD10, sample a shoulder at 350 ◦ C was also present. It is clear that the thermal evaporation of phenolphthalein occurred over at a higher and broader temperature range compared to pure phenolphthalein, providing support that phenolphthalein is complexed with the CD cavity. Additionally, the evolution of phenolphthalein over a broader temperature range may possibly be due to different environment/interactions of phenolphthalein with CD cavity. In brief, the DP-MS provides strong evidence that phenolphthalein was trapped by CD molecules on the surface of the PS/CD fibers. 4. Conclusion In the present study we produced beta-cyclodextrin functionalized electrospun polystyrene fibers (PS/CD) with the goal to develop functional fibrous membranes. XPS and ToF-SIMS analysis have provided valuable information about the surface chemistry of the PS/CD fibers produced by electrospinning, which is necessary for optimization of the experimental parameters for creating highly efficient molecular filters/nanofilters. XPS allowed for quantification of the surface concentration of CD in the fiber webs while ToF-SIMS provided supportive molecular information confirming the presence of CD at the surface. The PS/CD fibers are able to effectively remove a model organic compound (phenolphthalein) from solution by inclusion complexation with the surface associated -CD molecules, as determined by UV–vis spectroscopy and direct pyrolysis mass spectrometry studies. These findings are very promising and show the potential application for the CD functionalized fibrous webs which may be used as molecular filters and/or nanofilters for filtration/purification/separation purposes. Acknowledgements We gratefully acknowledge the funding to the current project NanoNonwovens from The Danish Advanced Technology Foundation, the collaboration with Fibertex A/S, and the Danish Research Agency for the funding to the iNANO center. References [1] A. Greiner, J.H. Wendorff, Electrospinning: a fascinating method for the preparation of ultrathin fibers, Angew. Chem. Int. Ed. 46 (2007) 5670–5703.
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Besenbacher, Electrospinning of functional poly(methyl methacrylate) (PMMA) nanofibers containing cyclodextrin– menthol inclusion complexes, Nanotechnology, in press. [27] M.A. Hunt, D.-W. Jung, M. Shamsheer, T. Uyar, A.E. Tonelli, Polystyrenes in channels, Polymer 45 (2004) 1345–1347. [28] T. Uyar, H.S. Gracz, M. Rusa, I.D. Shin, A. El-Shafei, A.E. Tonelli, Polymerization of styrene in gamma-cyclodextrin channels: lightly rotaxanated polystyrenes with altered stereosequences, Polymer 47 (2006) 6948– 6955. [29] T. Jarusuwannapoom, W. Hongroijanawiwat, S. Jitjaicham, L. Wannatong, M. Nithitanakul, C. Pattamaprom, P. Koombhongse, R. Rangkupan, P. Supaphol, Effect of solvents on electro-spinnability of polystyrene solutions and morphological appearance of resulting electrospun polystyrene fibers, Eur. Polym. J. 41 (2005) 409–421. [30] T. Uyar, F. Besenbacher, Electronspinning of uniform polystyrene fibers: the effect of solvent conductivity, Polymer 49 (2008) 5336–5343. [31] D. 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Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Molecular filters based on cyclodextrin functionalized electrospun fibers Tamer Uyar a,∗ , Rasmus Havelund a , Yusuf Nur c , Jale Hacaloglu c , Flemming Besenbacher a,b , Peter Kingshott a,∗∗ a
Interdisciplinary Nanoscience Center (iNANO), Aarhus University, DK-8000 Aarhus C, Denmark Department of Physics and Astronomy, Aarhus University, DK-8000 Aarhus C, Denmark c Department of Chemistry, Middle East Technical University, Ankara 06530, Turkey b
a r t i c l e
i n f o
Article history: Received 26 November 2008 Received in revised form 26 January 2009 Accepted 27 January 2009 Available online 6 February 2009 Keywords: Cyclodextrin Electrospinning Nanofiber Polystyrene Nanofilter
a b s t r a c t Beta-cyclodextrin (-CD) was successfully incorporated into polystyrene (PS) fibers by electrospinning technique. The subsequent fibrous membranes show potential for efficient removal of organic compound (e.g.: phenolphthalein) from solution by the formation of inclusion complexes with the -CD molecules. Since the filtration efficiency of the fiber membranes is highly dependent on the presence and distribution of -CD at the surface of the individual fibers, highly sensitive and specific surface spectroscopic analyses were carried out. X-ray photoelectron spectroscopy (XPS) was used to quantify the amount of -CD on the surface of PS fibers. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) showed the presence of both -CD molecular and fragment ions on the fiber surfaces, and high resolution chemical imaging demonstrated even distribution of -CD on the surface of the individual fibers. Phenolphthalein (PhP) was used as a model compound in the filtration studies, and a strong dependence was observed between -CD content of the PS/CD fibers and efficiency of trapping PhP. These results demonstrate the potential of using PS/CD fibrous membranes to filter organic molecules in purification/separation processes. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Electrospinning is a technique for producing fibers of micrometer and nanometer diameters by creating a continuous filament by exposing a polymer solution or polymer melt to very high electrical fields. Due to its versatility and cost effectiveness electrospinning has gained wide use recently to produce multi-functional nanofibers from different materials, which include polymers, polymer blends, ceramics, sol–gels and composite solutions with nanoparticles [1,2]. Nanofibers/nanowebs produced by electrospinning have several remarkable characteristics such as very large surface area to volume ratio, pore sizes in nanorange, unique physical and mechanical performance along with the design flexibility for chemical/physical surface functionalization. It has been shown that the outstanding properties and multi-functionality of such nanofibers/nanowebs make them favorable candidates for many applications in biotechnology, textiles, membranes/filters, composites, sensors, etc. [1–5].
∗ Corresponding author. Present address: UNAM-Institute of Materials Science & Nanotechnology, Bilkent University, Ankara 06800, Turkey. Tel.: +45 8942 3553; fax: +45 8942 3690. ∗∗ Corresponding author. Tel.: +90 312 2903571; fax: +90 312 2664365. E-mail addresses: [email protected], [email protected], [email protected] (T. Uyar), [email protected] (P. Kingshott). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.01.047
We report here a new method for taking advantage of the high surface-mass ratio of fibers to form molecular filters and/or nanofilters, with the added advantage of incorporating cyclodextrins (CDs) as functional additives that can trap molecules and function as highly efficient filtration systems. CDs are cyclic oligosaccharides consisting of 1,4-linked glucopyranoside units having either six, seven, or eight glucose units arranged in a cyclic structure, named as ␣-, - and ␥-cyclodextrins, respectively (Fig. 1). The hydrophobic cavity of CDs allows them to form noncovalent host–guest inclusion complexes (CD-ICs) with various molecules [6,7]. The formation of CD-IC and its stability depend on many factors such as the size/shape fit and binding forces between the host CD molecules and the guest molecules, and the chemical surroundings [8]. Since the physical and chemical properties of incorporated guest compounds can be tailored by complexation with CDs, the CDs are used in a variety of application areas, such as pharmaceuticals, foods, cosmetics, home/personal care, textiles, etc. [6,7]. Moreover, cyclodextrins and cyclodextrin functionalized materials are also used in filters and membranes for separation/purification/filtration purposes [9–13]. The electrospun nanofibrous mats can both physically filter tiny particles and remove industrial waste [5,14,15]. The combination of cyclodextrins and the electrospun nanofibers can potentially increase the efficiency of filters by facilitating complex formation with organic compounds and the very high surface area of the nanofibers. There are only a limited number of studies
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Fig. 1. (a) Chemical structure of -CD and (b) approximate dimensions of ␣-, -, ␥-CDs.
in the literature that report incorporation of CDs into electrospun fibers for various purposes [16–23]. For instance, -CD have been used to crosslink poly(acrylic acid) nanofibers in order to produce water-insoluble polyelectrolyte nanowebs [16]. The composite electrospun poly(N-vinylpyrrolidone) (PVP) nanofibers were obtained with -CD [17] and in a later study PVP nanofibers containing gold nanoparticles have been produced in which -CD was used as a stabilizing and reducing reagent [18]. Furthermore, a catalyst for the detoxification of nerve agents has been synthesized from -CD and o-iodosobenzoic acid. This modified -CD was incorporated into polyvinyl chloride (PVC) nanofibers in order to develop functional nanofibrous membranes that can provide protection from chemical warfare stimulant [19]. Poly(methyl methacrylate) (PMMA) nanofibers have been electrospun with phenylcarbomylated -CD in order to try to capture organic molecules for waste treatment [20]. In our recent studies we have produced poly(ethylene oxide) (PEO) nanofibers containing cyclodextrin/poly(ethylene glycol) (PEG) inclusion complex [21]. We have also successfully produced electrospun PMMA [22], PEO [23] and polystyrene (PS) nanofibers [24] functionalized with cyclodextrins. Furthermore, we incorporated CD–menthol inclusion complex in electrospun PS and PMMA nanofibers for the purpose to produce functional nanofibers that contain fragrances/flavors with high temperature stability [25,26]. In the present study we produced cyclodextrin functionalized electrospun polystyrene nanofibers (PS/CD) with the goal to develop functional nanowebs and we have shown that these CD functionalized nanowebs may have the potential to be used as molecular filters and/or nanofilters for filtration/purification/separation purposes. Polystyrene was chosen as a fiber matrix since PS does not form inclusion complexes with -CD as the cavity of -CD is too narrow to encapsulate atactic PS chains [27,28]. Thus, the cavity of -CD molecules will be empty and able to capture organic molecules. We studied the filtration process by using phenolphthalein (PhP) as a model organic molecule since it forms inclusion complexes with -CD and can be readily monitored by adsorption measurements. We investigated the surface composition and homogeneity of individual PS/CD fibers using highly surface sensitive spectroscopic techniques, X-ray photoelectron spectroscopy (XPS) and imaging time-of-flight sec-
ondary ion mass spectrometry (ToF-SIMS). These two techniques can give information on the surface compositions and spatial distribution of molecules on the surface of individual fibers. Such knowledge is highly desirable for understanding and optimizing the properties of modified fibrous materials. Finally, evidence of inclusion complexation of phenolphthalein by the CD molecules located on the fibers was confirmed both by UV–vis spectroscopy and direct pyrolysis mass spectrometry (DP-MS) studies. 2. Experimental 2.1. Materials Amorphous polystyrene (Mw ∼ 280,000), N,N-dimethylformamide (DMF) (99%), phenolphthalein (ACS reagent) and ethanol (absolute, HPLC grade, ≥99.8%) were purchased from Sigma–Aldrich. Beta-cyclodextrin (-CD) was a gift from Wacker Chemie AG (Germany). The materials were used without any purification. 2.2. Electrospinning The homogeneous clear solutions were prepared by dissolving PS and -CD in DMF at room temperature. The polymer concentration was varied from 15% to 25% (w/v) and the -CD content was varied from 10% to 50% (w/w) with respect to polymer. The polymer solutions were placed in a 1 mL syringe fitted with a metallic needle of 0.4 mm inner diameter. The syringe is fixed horizontally on the syringe pump (Model: KDS 101, KD Scientific), and a electrode of high voltage power supply (Spellman High Voltage Electronics Corporation, MP Series) was clamped to the metal needle tip. The flow rate of polymer solution was 1 mL/h and the applied voltage was 15 kV. The tip-to-collector distance was set to 10 cm and a grounded stationary rectangular metal collector (15 cm × 20 cm) covered by a piece of clean aluminum foil was used for the fiber deposition. The whole electrospinning apparatus was enclosed in glass box, and the electrospinning was carried out in a horizontal position at room temperature. The fibers collected on aluminum foil were dried at 40 ◦ C under vacuum oven for 24 h to remove the residual solvent.
Table 1 The properties of PS and PS–CD solutions and the resulting electrospun PS and PS/CD fibers. Materials
% PSa (w/v)
% CDb (w/w)
Fiber diameterc (nm)
Fiber morphology
Viscosity (cP)
PS10 PS15 PS20 PS25 PS20/CD10 PS15/CD25 PS10/CD50
10 15 20 25 20 15 10
– – – – 10 25 50
– – – 1959 ± 1610 ± 1214 ± 1161 ±
Beaded nanofibers Beaded nanofibers Beaded nanofibers Bead-free microfibers Bead-free microfibers Bead-free microfibers Bead-free microfibers
22.9 59.9 138.3 240.4 146.9 60.6 21.1
a b c
With respect to solvent (DMF). With respect to polymer (PS). Only bead-free fibers were reported.
162 186 279 271
± ± ± ± ± ± ±
0.1 0.5 0.7 1.5 0.7 0.2 0.3
Conductivity (S/cm) 2.5 1.35 1.1 1.1 2.4 3.8 5.3
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Fig. 2. SEM images of (a) PS25, (b) PS20/CD10, (c) PS15/CD25, and (d) PS10/CD50.
2.3. Measurements and characterization The viscosity of the polymer solutions was measured by using Brookfield DV-III Ultra Rheometer equipped with cone/plate accessory using the spindle type CPE-41. The viscosity measurements were repeated three times or more to ensure consistent viscosity readings. The conductivity of the solutions was measured with a Multiparameter meter InoLab® Multi 720 (WTW) at room temperature. The fiber morphology was examined by high resolution scanning electron microscopy (SEM) (FEI, Nova 600 NanoSEM) at 10 kV. The aluminum foil with the nanofibers was directly put into the SEM chamber without any metal sputtering or coating. The average fiber diameter was calculated from SEM images and around 50 fibers were measured. X-ray photoelectron spectroscopy was performed using a Kratos Axis UltraDLD instrument equipped with a monochromated Al K␣ X-ray source (h = 1486.6 eV) operating at 10 kV and 15 mA (150 W). The nanofiber samples that were analyzed were cut from the Al foil containing a thick layer of fibers. A hybrid lens mode was employed during analysis (electrostatic and magnetic), with an analysis area of approximately 300 m × 700 m. For each sample, a take-off angle (TOA) of 0◦ (with respect to the sample surface) was used allowing a maximum probe depth (10 nm). Wide energy survey scans (WESS) were obtained over the range 0–1400 eV binding energy (BE) at a pass energy of 160 eV, and used to determine the surface elemental composition. High resolution spectra were recorded for C 1s and O 1s at a detector pass energy of 20 eV. The Kratos charge neutralizer system was used on all samples with a filament current of between 1.8 and 2.1 A and a charge balance of 3.6 V. Sample charging effects on the measured BE positions were
corrected by setting the lowest BE component of the C 1s spectral envelope to 285.0 eV, corresponding to the C–C/C–H species. Deconvolution of the high-resolution spectral regions was performed by subtraction of a linear background and application of a mixed Gaussian–Lorentzian synthetic peak. Full width at halfmaximum (FWHM) values for each component were often set to values that gave the best fits and made the most physical sense in relation to the resolving power of the instrument and experimental parameters used. ToF-SIMS analysis was performed using an ION-TOF TOF.SIMS 5 instrument equipped with a Bi primary ion cluster source operating at 25 kV. In most cases Bi3 + primary ions were used with a target current of 0.3 pA. High-resolution mass spectra (M/M > 4000 at m/z = 27) were acquired using the high current bunched mode. High mass resolution spectra were obtained by the use of bunched primary ions. The analysis area was 500 m × 500 m. Only positive secondary ions spectra were acquired for in-depth analysis. Mass calibration of the spectra was based on CH3 + , C2 H3 + , C2 H5 + , and C3 H5 + ions. Chemical imaging was performed with the burst alignment mode which offers a high spatial resolution but low mass resolution. Due to the low mass resolution, chemical imaging was based only on peaks unambiguously identified from high mass resolution spectra over an area of 75 m × 75 m. In all cases an electron flood gun was for charge compensation and the primary ions dose was kept below 1012 ions/cm2 to stay within the static SIMS regime. The filtration performance of the PS and PS/CD fibers was tested using phenolphthalein as a model organic molecule. Uptake was determined by measuring the reductions in absorbance or depletion from solution using UV–vis spectrophotometry (Helios-) in the wavelength range of 400–700 nm. The weight of fiber mass
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Fig. 3. Deconvoluted XPS C 1s spectra for (a) the PS fibers, (b) pure -CD, (c) PS20/CD10 fibers, (d) PS15/CD25 fibers, and (e) PS10/CD50 fibers.
and the amount of PhP used were calculated under the assumption that 10% CD (w/w) is present on the fiber surface and thus available for complexation and 1:1 complexation between CD and PhP molecules occurs. A 4 × 10−4 M of PhP solution was prepared in absolute ethanol. The pH of the solution was adjusted to basic (above pH 11) by adding drops of pH buffer solution. The pH of the solutions was measured with a pH meter (pH meter 1500, EUTECH Instruments) before and after the UV–vis experiments; the pH of the solutions was unchanged during the course of the UV–vis experiments. 16 mg of PS and PS/CD fibers were placed separately in the bottom of the UV–vis cuvettes filled with PhP solution. The cuvettes were covered tightly with a Teflon lid to prevent the evaporation of ethanol and allowed to stand undisturbed 3 days. The absorbance spectra of the PhP solutions were recorded initially (time = 0, right after adding the fibers) and every 24 h. After 3 days the fibrous mats were removed, rinsed with ethanol and water to remove physically adsorbed PhP molecules, and analyzed by direct pyrolysis mass spectrometry. The DP-MS system consists of Waters Quattro MicroGC tandem MS with an EI ion source and a mass range of 10–1500 Da coupled with a direct insertion probe (Tmax = 650 ◦ C). 0.01 mg of fiber was pyrolyzed in flared quartz sample vials. The temperature was increased at a rate of 10 ◦ C/min and the scan rate was 1 scans/s, with simultaneous mass spectrometric analysis of the pyrolytic fragments. 3. Results and discussion
varied from 10% (w/w) to 50% (w/w) with respect to PS. The characteristics of PS and PS/CD solutions and the resulting electrospun fibers are summarized in Table 1. The scanning electron microscopy images of electrospun PS fibers and PS/CD fibers containing 10%, 25% and 50% (w/w) -CD are shown in Fig. 2. For PS solutions, beaded fiber structures were obtained at lower polymer concentrations (10–20%, w/v) but an increase in the polymer concentration to 25% (w/v) yielded bead-free fibers (Fig. 2a), which indicates that a high viscosity is required to obtain uniform PS fibers. This finding is consistent with previous findings in the literature where bead-free PS fibers were obtained only at the high concentration/viscosity range yielding microfibers [29,30]. On the contrary, the PS/CD systems yielded bead-free uniform fibers even at low concentrations of PS (10–20%, w/v) depending on the CD content. The positive effects of CD on electrospinning of bead-free PS fibers from low concentrations were discussed in detail in a previous publication [24]. In brief, 20% PS/10% -CD, 15% PS/25% -CD and 10% PS/50% -CD combinations yielded bead-free fibers. The viscosity of the PS/CD solutions was almost the same as the pure PS solutions indicating the negligible effect of -CD. However, the addition of the -CD increased the conductivity of polymer solutions, and resulted in large differences in fiber morphology of fibers that were bead-free. It is also worth noting that the average diameter of the PS/CD fibers was narrower since low PS concentrations were used. Although the average fiber diameters for the PS and PS/CD electrospun systems are in micron range, nanofibers from these systems can be easily achieved by increasing the conductivity of the solution, that is, by adding small amount of salt to the polymer solutions [30].
3.1. SEM characterization 3.2. XPS and ToF-SIMS characterization The optimization of the electrospinning conditions for producing uniform cyclodextrin functionalized PS fibers (PS/CD) and the morphological and bulk chemical characterization of the PS/CD fibers have been reported previously [24]. The concentration of the PS solutions was varied from 10% (w/v) to 25% (w/v), and -CD was
Of particular interest are the surface properties of the PS/CD since they determine the functionality of filter, and during electrospinning it is quite possible that some CD molecules may phase separate from the PS matrix and reside on the surface of the fibers.
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Table 2 Atomic concentrations generated from XPS wide energy survey scans. Sample
C (%)
O (%)
PS -CD PS20/CD10 PS15/CD25 PS10/CD50
100 58.7 98.8 98.0 94.3
0 41.3 1.2 2.0 5.7
XPS and ToF-SIMS analyses were performed on the different PS/CD fibers in order to verify both the amount of CD present on the surfaces (XPS), and the lateral distribution of CD molecules present on individual PS/CD fibers (ToF-SIMS). This is important if the CD molecules are to be used to capture molecules from solvents that do not swell the PS matrix, since they will only be exposed to the surface. Table 2 shows XPS atomic concentrations generated from wide energy survey scans for PS fibers, pure -CD, and PS/CD fibers. The data clearly demonstrate that the surface of the PS/CD fibers is mainly PS with small amounts of CD present as indicated by the low oxygen content compared to pure CD. An increase in oxygen content is observed for increasing concentration of CD in the PS solution. The measured probe depth under the experimental conditions chosen is ∼10 nm and the spot size is 700–300 m, thus an average surface composition is provided. The chemical functionality of the electrospun fibers was evaluated from the core level C 1s spectra as shown in Fig. 3 PS fibers, -CD and three different PS/CD fibers. For the pure PS fibers the peaks assigned as follows [31]: C C of the benzene ring of PS at 284.6 eV (peak C), the aliphatic carbon atoms of PS at 285.0 (peak B) and the –* shake-up satellites (peak A) around 291.5 eV. The pure -CD spectrum (Fig. 3b) shows three peaks that are assigned to C–C/C–H (small hydrocarbon contamination) (peak B), C–O–C/C–OH at 286.7 eV (peak D) and O–C–O/C O at 288.1 eV (peak E). The C 1s spectra for PS/CD fibers (Fig. 3c–e) are a composition of peaks from PS and CD, with PS peaks as the major components. A peak D at 286.4–286.8 eV is assigned to the C–O–C/C–OH species of -CD [32]. This peak component becomes more prominent with increasing -CD concentration, which is consistent with the elemental composition. The O–C–O/C O component in the pure -CD (peak E) is only visible for PS10/CD50. Based on the data in Table 2 it is estimated that about 3%, 5%, and 13% of the probed volume consists of -CD molecules for the PS20/CD10, PS15/CD25, and PS10/CD50 samples, respectively. The surface CD content for all three PS/CD samples is substantially lower than the CD content of the solutions they were prepared from, indicating that most the -CD molecules are buried in the bulk of the fibers. Thus, optimization of the experimental parameters to obtain high CD content on the fiber surfaces is desirable to improve the filtration efficiency of these fibrous membranes. With a depth resolution of 1–2 nm, ToF-SIMS was used to obtain molecular information of the PS/CD fiber surfaces, and assess the uniformity of the -CD distribution on individual PS/CD fibers surface. Initially spectra were acquired in high mass resolution mode to verify the presence of -CD at the surface of the samples through the identification of specific fragment ions. The positive ion spectrum for PS15/CD25 is shown in Fig. 4, and the assignments for selected ions are shown in Table 3 with accurate mass determination. The same peaks are observed in the spectra for the two other PS/CD fiber samples, thus making the assignment valid for all samples. The spectrum of pure PS (not shown) shows a specific pattern of peaks which can all be assigned to Cx Hy + ions with the most intense being the tropylium ion, C7 H7 + at m/z = 91. These peaks also appear in the PS/CD spectra. Both CD and PS can fragment into Cx Hy + ions
Fig. 4. Representative positive ion ToF-SIMS spectrum recorded from the surface of PS15/CD25 fibers. Major peaks assignments are shown in Table 3.
when x ≤ 5 and, thus, assignments of peaks to either PS or CD must be carried out carefully in this range. At higher mass, a number of peaks unambiguously identify -CD (see Table 3). The presence of -CD is most characteristically indicated by a peak at m/z = 1157, which is assigned to the single charged Na-adduct ion of the intact -CD molecule. Peaks at masses m/z = 163 and 325 correspond to the fragment ions of -CD assigned to one and two glucopyranoside units, respectively. A number of peaks in the lower mass range can be assigned to oxygen containing fragment ions. The high mass resolution allows for separation of these ions from the Cx Hy + ions originating from polystyrene. Table 3 Peak assignments derived from the positive ion TOF-SIMS spectra of PS–CD. Peak
Ion
Theoretical mass
Measured mass
77 91 103 117 193 69 73 81 85 87 97 101 163 325 1157
C6 H5 + C7 H7 + C8 H7 + C9 H9 + C15 H13 + C4 H5 O+ C3 H5 O2 + C5 H5 O+ C4 H5 O2 + C4 H7 O2 + C5 H5 O2 + C4 H5 O3 + C6 H11 O5 + C12 H21 O10 + C42 H70 O35 Na+
77.0391 91.0548 103.0548 117.0704 193.1017 69.0340 73.0290 81.0340 85.0290 87.0446 97.0290 101.0239 163.0606 325.1135 1157.3595
77.0412 91.0571 103.0504 117.0713 193.1025 69.0349 73.0356 81.0385 85.0345 87.0518 97.0284 101.0281 163.0749 325.1657 1157.3710
The peaks are used for chemical imaging shown in Fig. 5.
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Fig. 5. ToF-SIMS chemical images of fibers taken using the burst alignment mode. (a) Total ion image of PS15/CD25, (b) PS fragment ion image of PS15/CD25, and (c) CD fragment ion image of PS15/CD25. The distribution of CD on the surface of different PS/CD fibers shown by overlays of CD (green) on PS (red) fragment ions image, respectively (software color). (d) PS20/CD10, (e) PS15/CD25, and (f) PS10/CD50. Image area 75 m × 75 m. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
The fragment ions shown in Table 3 originate from either -CD or PS, and all appear at unique m/z values. These peaks are used to derive 2D chemical maps showing the distribution of either PS or CD molecules on the fiber surfaces. In this case the burst alignment mode of the ToF-SIMS instrument is used obtain high spatial resolution images, and to improve greater contrast either PS or -CD fragment ions by summing the intensity of the respective peaks. Fig. 5a–c shows secondary ion images of PS15/CD25 fibers over a 75 m × 75 m scan area. The respective images for PS and -CD are combined to form a single overlay image showing the distribution of CD on the surface of the fibers (Fig. 5e). Similar overlay images for the PS20/CD10 (Fig. 5d) and PS10/CD50 (Fig. 5f) are also shown. Clearly the individual fibers can be imaged by the contrast in the chemical composition between the PS and CD. The dominant features in the spectra are assigned to PS fragments indicating that PS is the major component on the surface, supporting the XPS data. However, all of the PS/CD samples contain regions on the fibers that show CD molecules. Some of these regions (green parts of Fig. 5) indicate aggregation of CD at the surface of the fibers. There are also some overlapping regions on the individual fibers, showing the presence of both PS and CD molecules and indicative of partial mixing between the two species. The localized distribution of
Table 4 The cumulative % decrease of PhP absorbance over time at 562 nm. Materials
PS25 PS20/CD10 PS15/CD25 PS10/CD50
Time (h) 0
24
48
72
0 0 0 0
11.64 10.41 28.33 35.2
12.30 24.25 42.41 53.64
11.37 33.29 52.88 63.25
CD at the surface of the fibers is seen in all PS–CD samples. The relative amount of CD within the larger CD clusters and the actual size of the clusters increases with increasing CD concentration. The observed distribution behavior will play a large role in determining the efficiency of the fibers as filters, and experiments are underway to optimize the electrospinning process. The improvement of the CD content on the fiber surfaces and the uniform dispersion of the CD may be achieved by varying the spinning conditions and parameters such as electrospinning at high temperature and/or using different solvent/co-solvent systems. An alternative approach might be to use chemically modified CD derivatives which tend to phasesegregate to the fiber surface during the solvent evaporation under the applied electrospinning process. 3.3. Uptake of phenolphthalein The trapping ability of -CD functionalized PS fibers was tested using phenolphthalein as a model system. PhP has often been used as a standard to test for CD inclusion complexation due to its high affinity for the CD cavity [33,34]. The PS and PS/CD fibers were immersed into a PhP solution and the change in absorbance of PhP was recorded as a function of time by UV–vis spectrometry (Fig. 6). It was observed that the absorbance of PhP solution decreased significantly over time in the presence of PS/CD fibers and pink solution become colorless, due to the removal of PhP from solution by -CD. Table 4 summarizes the cumulative % decrease of PhP absorbance over time. The PS/CD fibers function as a molecular filter through complexation of the PhP molecules with the -CD molecules on the surface of the PS fibers. For PS fibers, no color change was observed and only a small decrease (∼12%) in PhP absorbance was observed after the first day and stayed more or less constant afterwards. The small decrease in PhP concentration is most likely due to adsorption onto the surface of the fibers. The slight absorbance increase in
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Fig. 6. UV–vis spectra of phenolphthalein (PhP) solution as a function of time after dipping the webs of (a) PS25, (b) PS20/CD10, (c) PS15/CD25, and (d) PS10/CD50.
the third day is possible due to desorption of some PhP molecules from the fiber surface. On the other hand, for the PS/CD fibers, the decrease in PhP absorption was significant after the first day (in the range of ∼10–35% depending on the CD content), with continual decreases reaching ∼35–65% after 3 days. The decrease for PhP was about constant after 6 days (∼35%) for PS/CD10 fibers indicating that the saturation point was reached after approximately 3 days. It was observed that the higher the content of CD on the fibers, the more rapidly the PhP is removed from the solution (Table 4). These findings are very promising and show the potential application for cyclodextrin functionalized electrospun fibers to be used in filters for the removal of organic molecules. Ongoing studies are investigating the ability of these CD functionalized PS fibers at capturing organic molecules (e.g.: styrene, aniline and toluene) from the solutions. CD molecules have the ability to trap polluting substances (organic waste, heavy metals, radioactive wastes, etc.) from the environment [9–13] where the complexation is limited by the size of the waste molecules and CD cavity. We expect that such cyclodextrin functionalized electrospun nanofilters will be very applicable for waste treatments as long as the CD cavity can form complexation with the target molecules.
detection of a peak, but also the variation of its intensity as a function of temperature, i.e., evolution profile, is important. The trends in the evolution profiles can be used to determine the source of the product, or the mechanism of thermal degradation.
3.4. Direct pyrolysis mass spectrometry In order to confirm the complexation of -CD with phenolphthalein molecules, direct pyrolysis mass spectrometry analysis was performed on the PS/CD fibers after exposed to PhP solution. For a multi component system, DP-MS allows separation of components as a function of their volatilities and/or thermal stabilities. Once included in the host CD cavities, the thermal evaporation/decomposition of the guest molecules shifts to higher temperatures due to the strong interaction with the CD cavity, thus, DP-MS technique is a useful technique to characterize the CD host–guest inclusion complexes [35]. In general, DP-MS facilitates analyses of volatility, thermal stability and thermal degradation products of materials which can be used for investigation of chemical structures [36,37]. In pyrolysis MS analysis, not only the
Fig. 7. DP-MS evolution profiles of PS-based product; monomer (m/z = 104 Da), phenolphthalein (m/z = 318 Da) and CD-based product; C2 H4 O2 (m/z = 60 Da) detected during the pyrolysis of (a) pure samples (PS fibers, phenolphthalein and b-CD), (b) PS25, (c) PS20/CD10, (d) PS15/CD25, and (e) PS10/CD50. (Note: The fibers were analyzed after the exposure to PhP solution at the end of the UV–vis experiments.)
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PS and PS/CD fibers exposed to phenolphthalein solution were analyzed by DP-MS to investigate the presence of phenolphthalein and its thermal stability in the samples. DP-MS evolution profiles of PS fiber, phenolphthalein, -CD and PS/CD fibers are shown in Fig. 7. The evolution profiles of some diagnostic products of polystyrene, phenolphthalein and -CD namely; the styrene monomer (m/z = 104 Da), phenolphthalein (molecular peak, m/z = 318 Da) and C2 H4 O2 (m/z = 60 Da) respectively, detected during the pyrolysis are shown. The evolution of polystyrene and CD-based products generated during the pyrolysis of PS and PS/CD fibers exposed to phenolphthalein solution showed almost identical trends with the corresponding profiles detected during the pyrolysis of pure PS and CD. The pyrolysis of pure phenolphthalein started just above 170 ◦ C, maximizing around 257 ◦ C and completed around 280 ◦ C (Fig. 7a). In the case of pure PS fibers (Fig. 7b), the phenolphthalein was hardly detected, confirming that only a small amount of was adsorbed on the surface and washed out during the rinsing process. This correlates well with UV–vis absorbance results as discussed previously. On the other hand, all the evolution profiles of PS/CD fibers showed the presence of phenolphthalein. Moreover, the evolution of phenolphthalein shifted to higher temperatures during the pyrolysis of these samples (Fig. 7c–e); phenolphthalein evolution was maximized at 300 ◦ C for PS20/CD10 and PS10/CD50 samples, and at 350 ◦ C for PS15/CD25 sample. For the PS10/CD50 sample, a second maximum and, for the PS20/CD10, sample a shoulder at 350 ◦ C was also present. It is clear that the thermal evaporation of phenolphthalein occurred over at a higher and broader temperature range compared to pure phenolphthalein, providing support that phenolphthalein is complexed with the CD cavity. Additionally, the evolution of phenolphthalein over a broader temperature range may possibly be due to different environment/interactions of phenolphthalein with CD cavity. In brief, the DP-MS provides strong evidence that phenolphthalein was trapped by CD molecules on the surface of the PS/CD fibers. 4. Conclusion In the present study we produced beta-cyclodextrin functionalized electrospun polystyrene fibers (PS/CD) with the goal to develop functional fibrous membranes. XPS and ToF-SIMS analysis have provided valuable information about the surface chemistry of the PS/CD fibers produced by electrospinning, which is necessary for optimization of the experimental parameters for creating highly efficient molecular filters/nanofilters. XPS allowed for quantification of the surface concentration of CD in the fiber webs while ToF-SIMS provided supportive molecular information confirming the presence of CD at the surface. The PS/CD fibers are able to effectively remove a model organic compound (phenolphthalein) from solution by inclusion complexation with the surface associated -CD molecules, as determined by UV–vis spectroscopy and direct pyrolysis mass spectrometry studies. These findings are very promising and show the potential application for the CD functionalized fibrous webs which may be used as molecular filters and/or nanofilters for filtration/purification/separation purposes. Acknowledgements We gratefully acknowledge the funding to the current project NanoNonwovens from The Danish Advanced Technology Foundation, the collaboration with Fibertex A/S, and the Danish Research Agency for the funding to the iNANO center. References [1] A. Greiner, J.H. Wendorff, Electrospinning: a fascinating method for the preparation of ultrathin fibers, Angew. Chem. Int. Ed. 46 (2007) 5670–5703.
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