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Functional-group analysis of polyvinylpyrrolidone on the inner surface of hollow-fiber dialysis membranes, by near-field infrared microspectroscopy
Journal of Membrane Science 355 (2010) 208–213
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Functional-group analysis of polyvinylpyrrolidone on the inner surface of hollow-fiber dialysis membranes, by near-field infrared microspectroscopy Sumire Koga a , Taiji Yakushiji b , Masato Matsuda a , Ken-ichiro Yamamoto c , Kiyotaka Sakai a,∗ a b c
Department of Chemical Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan Department of Medical Engineering, Himeji Dokkyo University, 7-2-1 Kamiono, Himeji, Hyogo 670-8524, Japan Waseda Institute for Advanced Study, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo 169-8050, Japan
a r t i c l e
i n f o
Article history: Received 28 January 2010 Received in revised form 12 March 2010 Accepted 20 March 2010 Available online 27 March 2010 Keywords: NFIR (near-field infrared microspectroscopy) Dialysis membrane Surface characterization Polyvinylpyrrolidone (PVP)
a b s t r a c t Near-field infrared microspectroscopy (NFIR) is a newly developed surface analysis method that is based on functional-group analysis and has a high spatial resolution. The objective of the present study is to perform nanoscale functional-group analysis of dialysis membrane surfaces by using NFIR. We focused on polyvinylpyrrolidone (PVP), which is employed as an additive to hydrophilize and create pores in synthetic polymer dialysis membranes, and evaluated the PVP distribution on the inner surface of the dialysis membranes. Dialysis membranes made from polysulfone (PSf) and polyester–polymer alloy (PEPA) were first assessed by NFIR and then by attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR). The nanoscale NFIR analysis showed heterogeneous distribution of PVP on the PSf membrane even though the amount of PVP on the membrane surface was high. PVP was homogeneously distributed on the PEPA membrane even though the amount of PVP on the membrane surface was low. In contrast, the microscale ATR-FTIR results showed that PVP was homogeneously distributed on both PSf and PEPA membranes. PVP-rich and PVP-poor regions were distinguishable by NFIR and not by ATR-FTIR, because the spatial resolution of NFIR is higher than that of ATR-FTIR. This study demonstrates for the first time that NFIR can provide nanoscale chemical information on the structures of porous membranes. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent times, techniques that employ near-field light (NFL) have attracted much attention. NFL is a localized electromagnetic field that lies close to a material surface. Therefore, unlike propagating light, NFL shows no limitations of spatial resolution resulting from light diffraction. NFL has been used to develop an optical microscope [1] with high spatial resolution and a light record system [2] capable of ultra-density recording. NFL-based techniques are expected to be used for various applications such as Raman spectroscopy and polarization microscopy [3–5]. Near-field infrared microspectroscopy (NFIR) [6–8] is a surface analysis spectroscopic technique that employs NFL. Fig. 1 shows the principle of NFIR. The probe, on approaching the sample, is illuminated and NFL is generated around the probe apex. This NFL is scattered by mutual interactions such as absorption and reflection with the sample. The infrared absorption spectrum is obtained by analyzing the scattered light. The distinctive feature of NFIR is its excellent spatial resolution; nanoscale spot sizes can be obtained using NFIR. In contrast, the spatial resolution of con-
∗ Corresponding author. Tel.: +81 3 5286 3216; fax: +81 3 3209 7957. E-mail address: [email protected] (K. Sakai). 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.03.032
ventional infrared spectroscopy is restricted; a spot size below 10 m cannot be obtained with the conventional approach. Therefore, infrared spectroscopy is a microscale analysis method, and this spectroscopic method is incapable of performing functional-group analysis within a 10-m area. However, the spot size obtained by NFIR is the same as the size of the generated NFL which is equal to the radius of curvature of the probe apex; therefore, nanoscale NFL can be obtained by manufacturing a nanoscale probe. Thus, NFIR allows nanoscale analysis of the functional groups. NFIR is a useful analysis technique for obtaining submicroscale to nanoscale chemical information. It also enables the analysis of the functional groups of substances with submicron molecular sizes, which cannot be detected by conventional infrared spectroscopy. Thus, this method can yield chemical information of biomolecules such as proteins. It also enables identification of impurities with submicron sizes and determination of the locations of impurities on sample surfaces. Furthermore, NFIR can elucidate substance distribution by facilitating mapping measurements on 10-m and below areas, which cannot be assessed by conventional infrared spectroscopy. NFIR is a powerful tool for functional-group analysis of nanoscale sample surfaces. Although few studies have attempted to use NFIR for samples that are easy to prepare, such as meteorites [6] and particles [7,8], NFIR for soft synthetic organic materials has not been performed to
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Fig. 2. Chemical structural formula of PSf, PEPA and PVP.
Fig. 1. Principle of near-field infrared microspectoscopy.
date. Because the light spot obtained by NFIR is smaller than that obtained by conventional infrared spectroscopy, the surface roughness and surface thickness hinder the illumination and detection of light, thereby resulting in insufficient light detection. Therefore, samples with coarse surfaces and soft samples that are not sliced are difficult to analyze by NFIR. If such samples can be analyzed by NFIR, this technique can be used for a wide variety of samples. Thus far, we have analyzed dialysis membrane surfaces by both atomic force microscopy (AFM) [9–11] and attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) [12]. Dialysis membranes with adequate diffusive permeability and somewhat biocompatibility are used for the removal of uremic toxins in patients with renal failure [13,14]. Surface analysis [15–18] is essential to evaluate the structure and physicochemical properties of dialysis membrane surfaces, which affect their solute-removal performance and biocompatibility. Nanoscale evaluation is especially important for surface analysis of dialysis membranes. The nanoscale surface structure and physicochemical properties mainly affect the solute-removal performance and biocompatibility of the membrane, because dialysis membranes are nanoscale porous membranes, and mutual interactions at a molecular level cause protein adsorption onto membrane surfaces. However, conventional surface analyses such as AFM and ATR-FTIR cannot provide any nanoscale chemical information on dialysis membrane surfaces. AFM is capable of high-resolution evaluation of the physical structure and physicochemical properties of a material surface. Although AFM affords nanoscale observation of the surface structure, it cannot easily provide chemical information such as chemical composition and functional-group identity. In contrast, ATR-FTIR can be used to analyze the functional groups present on the material surface. However, ATR-FTIR does not provide submicroscale spatial resolution; a light collection system that uses propagating light cannot provide spatial resolution smaller than a wavelength because of the diffraction limit of illuminating light. For example, infrared microspectroscopy [19–23] combined with infrared spectrometry and microscopy provides a spatial resolution of about 10 m.
Therefore, conventional ATR-FTIR cannot provide microscale to nanoscale chemical information of the material surface. Thus, conventional analysis methods such as AFM and ATR-FTIR cannot provide molecule-level information of the chemical composition on the material surface. The objective of the present study is to employ NFIR to perform nanoscale functional-group analysis of dialysis membrane surfaces. We focused on polyvinylpyrrolidone (PVP), which is used as an additive to hydrophilize membrane surfaces and create pores in synthetic polymer dialysis membranes; we evaluated the PVP distribution on the inner surface of dialysis membranes. PVP increases the hydrophilicity of dialysis membranes and plays an important role in their biocompatibility. The membrane is expected to show excellent biocompatibility if PVP shows homogeneous nanoscale distribution on the dialysis membranes. Therefore, it is essential to examine the nanoscale PVP distribution on dialysis membranes. However, such examinations cannot be easily performed using AFM or by elemental analysis using X-ray photoelectron spectroscopy (XPS), because the membrane material is a polymer, and PVP is either mixed with the membrane material or modified on the membrane. Therefore, we used NFIR to analyze PVP distribution on membrane surfaces. Although many synthetic polymer dialysis membranes are commercially available, we used dialysis membranes made from polysulfone (PSf) and polyester–polymer alloy (PEPA), which are representative synthetic polymer dialysis membranes that have been used in Japanese medical institutions. We then tested the same membranes by conventional ATR-FTIR to evaluate the efficacy of NFIR and compared the results of ATR-FTIR with those of NFIR. 2. Materials and methods 2.1. Preparation of membrane samples for ATR-FTIR and NFIR analyses Table 1 shows the technical data on the four commercially available dialysis membranes tested in this study. Fig. 2 shows the structural formulas of PSf, PEPA, and the additive PVP. Four samples were prepared using the dialyzers listed in Table 1 and the following protocol. A hollow-fiber dialysis membrane taken out of the dialyzer was cut to obtain a small fiber sample; then, the central part of the membrane was longitudinally cut open to expose its inner
Table 1 Technical data on the dialysis membranes tested. Sample
Dialyzer
FLX FDX APS0 APSE
FLX-15GW FDX-150GW APS (prototype) APS-15E
Membrane material
Manufacturer
PEPA
Nikkiso
PSf
Asahi Kasei Kuraray Medical
PVP − + − +
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the curvature radius of the probe apex; therefore, nanoscale spatial resolution can be obtained by NFIR. FTIR is used for efficient spectral measurement. The scattered light from mutual interactions between the sample and probe, such as absorption and reflection, was detected. The detected light signal is transformed to a spectrum by Fourier transformation. Although NFL has excellent spatial resolution, the wave number of NFL is the same as that of propagating light. Therefore, the spectrum obtained by NFIR can be analyzed like the spectrum obtained by conventional ATR-FTIR.
Fig. 3. Mapping measurement by ATR-FTIR and NFIR.
surface. The cut edge was pressed by a pincette to fix the opened sample on a sample stage. These prepared test samples were dried at 333 K for 12 h. 2.2. Surface analysis by ATR-FTIR 2.2.1. Spectral measurement of the dialysis membrane surface The spectra of the inner surface of the dialysis membrane were measured by conventional ATR-FTIR. A multi-channel infrared microscope (IRT-7000; JASCO, Tokyo) was attached to an FT-IR Spectrometer (FT/IR-6200; JASCO, Tokyo), and this spectrometer was used as an infrared spectrometer. The spectra were mapped in a 500 m × 1200 m area on the sample surface with measuring points of 40 × 96 (Fig. 3). The number of spectral accumulations was 64 per point. The PVP content ratio for each point was calculated from the obtained spectrum.
2.3.2. Spectral measurement of PVP-coated glass surface PVP (1.0 g; PLASDONE K90; International Specialty Products Inc., New Jersey) was dissolved into 50 ml ethanol to prepare a PVP solution. A cover glass (24 mm × 36 mm; MATSUNAMI, Osaka) was soaked in the PVP solution and then dried at room temperature to prepare a PVP sample. A control sample was prepared without soaking the cover glass in the PVP solution. The spectra of the PVP-coated sample were measured on a near-field infrared microspectroscope (NFIR-200; JASCO, Tokyo). The number of spectral accumulations was 1000 per point. 2.3.3. Spectral measurement of dialysis membrane surface The spectra of the inner surface of the dialysis membrane were measured by NFIR. A near-field infrared microspectroscope (NFIR200; JASCO, Tokyo) equipped with a probe (curvature radius of the probe apex: 990 nm) was used for spectral measurement. The spectra were mapped in a 3 m × 3 m area on sample surface with measuring points of 4 × 4, as shown in Fig. 3. The number of spectral accumulations was 2000 per point. The PVP content ratio for each point was calculated from the obtained spectrum. 3. Results and discussion
2.3. Surface analysis by NFIR
3.1. Surface analysis by ATR-FTIR
2.3.1. Instrumentation Fig. 4 shows a schematic diagram of a near-field infrared microspectroscope (NFIR-200; JASCO, Tokyo). A near-field infrared microspectroscope consists of two systems: a stage system for distance control between the probe and the sample and a spectroscopic system for generation of NFL and detection of scattered light. The stage system uses the same system as AFM and is capable of subnanoscale control of distance and location. In the spectroscopic system, the probe apex (curvature radius of the probe apex: 990 nm) is illuminated by the spectroscopic light to generate NFL around the probe apex. The size of the generated NFL is the same as
3.1.1. Spectral identification Fig. 5 shows the ATR-FTIR spectra of the PEPA and PSf membranes. All the four samples showed a peak at 1580 cm−1 . This peak is derived from a benzene ring present in both PEPA and PSf membranes. Peaks at 1670–1680 cm−1 were obtained for FDX and APSE containing PVP. This peak is derived from the ␥-lactam present in PVP. 3.1.2. Microscale analysis of the dialysis membrane surface From the spectral data, the ratio of the peak areas of PVP and the polymer membranes was calculated to obtain a PVP content ratio, and two-dimensional images of the PVP content ratio were obtained (Fig. 6). The magnitude of the PVP content ratio was depicted using different colors depending on the PVP content ratio indicated in the right-hand side vertical scale. We evaluated the PVP distribution by using the PVP content ratio instead of the PVP peak area to eliminate sample errors ascribable to differences in the adhesion of the ATR-FTIR prism with the membrane surface. The presence of an air layer between the ATR-FTIR prism and membrane surface lowers the PVP peak area. The PVP content ratio of FLX and APS0 was 0. Although the PVP content ratio of FDX was lower than that of APSE, PVP was distributed homogeneously in both samples. 3.2. Surface analysis by NFIR
Fig. 4. Schematic diagram of near-field infrared microspectroscope.
3.2.1. Spectral analysis of PVP-coated glass surface Fig. 7 shows the spectra of the PVP-coated glass surface and untreated glass surface (control sample). The obtained spectra were treated by Kramers–Kronig (KK) transformation to convert transmittance into a complex refraction index k. In the case of NFIR,
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Fig. 7. PVP-coated glass surface spectra by NFIR.
Fig. 5. Dialysis membrane surface spectra by ATR-FTIR.
the probe apex was exposed to light to generate NFL around the apex. When the sample approaches the probe apex, the NFL is scattered via mutual interactions between the sample and the probe apex. Because NFIR detects the scattered light, the obtained spectra are essentially reflection spectra. The KK transformation is a method for analyzing reflection spectra and arithmetically comparing reflection spectra with transmission and absorption spectra. Peaks at 1671 cm−1 and 1243 cm−1 were observed for the PVPcoated glass surface (Fig. 7). A peak at 1228 cm−1 was observed for the glass surface. Peaks ranging from 1228 cm−1 to 1243 cm−1 , which were observed for both samples, were derived from the glass, and a peak at 1671 cm−1 was derived from the PVP. These results indicate that the wave number of the PVP peak observed in NFIR is almost similar to that of the PVP peak observed in ATR-FTIR.
Fig. 8. Dialysis membrane surface spectra by NFIR.
3.2.2. Nanoscale analysis of the dialysis membrane surface Fig. 8 shows the spectra of the PEPA and PSf membranes. The spectral data were transformed by the KK transformation from transmittance into the complex refraction index k. Peaks derived from PVP were observed at around 1670 cm−1 for both FDX and APSE. All four samples showed peaks at around
Fig. 6. PVP content ratio of dialysis membrane surface by ATR-FTIR (two-dimensional distribution).
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Fig. 9. PVP content ratio of dialysis membrane surface by NFIR (two-dimensional distribution).
1580 cm−1 . The latter peaks are derived from a benzene ring present in both PEPA and PSf membranes. The spectral data were used to calculate the peak area ratios of PVP and the polymer membrane and determine the PVP content ratio, and two-dimensional images of the PVP content ratio were obtained (Fig. 9). The magnitude of the PVP content ratio was depicted using different colors depending on the PVP content ratio indicated in the righthand side vertical scale. We evaluated the PVP distribution using the PVP content ratio instead of the PVP peak area to eliminate sample errors caused by differences in membrane surface roughness, which lowers the PVP peak area. The PVP content ratio of FLX and APS0 was 0. FDX and APSE showed heterogeneous PVP distribution, with the PVP-rich and PVP-poor regions forming a mosaic. Fig. 10 shows the average, maximum, and minimum values for the PVP content ratios of FDX and APSE. The PVP content ratio for APSE was high but the overall PVP content ratio for FDX was low, and the membrane had a few regions with no PVP. In our previous study, in which the adsorbability of human serum albumin (HSA) on the inner surface of a dialysis membrane [11] was measured by AFM, a local region showing high HSA adsorbability was observed in FDX. These results indicate the presence of some regions with no PVP, which corresponds to the results obtained by NFIR. Standard deviation indicates that the dispersion of the PVP content ratio for APSE is higher than that for FDX. Thus, although the quantity of PVP on the membrane surface of APSE is higher than that on the membrane surface of FDX, PVP homogeneity for APSE is lower than that for FDX. NFIR measurement is a nanoscale functional-group analysis because the spot size is 990 nm. These results demonstrate that NFIR can be used to obtain nanoscale chemical information on dialysis membrane surfaces.
3.3. Comparison of the measurement results obtained by ATR-FTIR with those obtained by NFIR Fig. 11 shows the average, maximum, and minimum ATR-FTIR and NFIR values for the PVP content ratios of FDX and APSE. The NFIR results are valid by the lack of significant differences in the average PVP content ratios obtained using conventional ATR-FITR and NFIR. However, the difference between the maximum and minimum PVP content ratios obtained using ATR-FTIR and NFIR was different from the corresponding value obtained using NFIR; the difference for NFIR was higher than that for ATR-FTIR. These results indicate that the PVP-rich and PVP-poor regions are distinguish-
Fig. 10. PVP content ratio of dialysis membrane surface by NFIR (average, maximum, minimum).
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Fig. 11. Comparison of PVP content ratio results by ATR-FTIR with those by NFIR (average, maximum, minimum) (n = 80: 16 measuring points × 5 times).
able by NFIR, because the spatial resolution of NFIR is much higher than that of ATR-FTIR. In the conventional microscale functionalgroup analysis by ATR-FTIR, each dialysis membrane was found to be homogeneously covered by PVP particles, and there was no difference in the PVP distributions of the PEPA and PSf membranes. However, nanoscale functional-group analysis by NFIR, which is capable of analyzing an area smaller than the spot size obtained by ATR-FTIR, demonstrates that PVP is heterogeneously distributed on dialysis membranes, and PVP homogeneity for the PEPA membrane is higher than that for the PSf membrane. In addition, NFIR analysis revealed regions on the PEPA membrane where the membrane material was partially exposed due to a lack of PVP on the PEPA membrane. Therefore, surface analysis by NFIR is effective for exact evaluation of PVP distribution on dialysis membrane surfaces. 4. Conclusions Although conventional microscale measurement shows homogeneous distribution of PVP on the surface of dialysis membranes, nanoscale analysis by NFIR shows heterogeneous distribution of PVP on the membrane surface and differences in the PVP homogeneities of different dialysis membranes. In addition, NFIR reveals regions where the membrane material is partially exposed due to a lack of PVP on the PEPA membrane. The present study demonstrates for the first time that NFIR can be used for nanoscale assessments of the structure of porous membranes. Acknowledgment This study was partly supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. References [1] V.F. Dryakhlushin, V.P. Veiko, N.B. Voznesenskii, Scanning near-field optical microscopy and near-field optical probes: properties, fabrication, and control of parameters, Quantum Electron. 37 (2) (2007) 193–203. [2] K. Kurihara, Y. Yamakawa, T. Shima, J. Tominaga, Capacity increase in radical direction of super-resolution near-field structure read-only-memory disc, Jpn. J. Appl. Phys. 43 (6B) (2007) 3898–3901. [3] T. Inoue, F. Sato, Y. Narita, Near-field fiber probe for polarization spectroscopy, Vib. Spectrosc. 35 (2004) 33–37. [4] T. Saiki, Y. Narita, Nano-optical imaging spectroscopy: recent advances in nearfield scanning optical microscopy, JSAP Int. 5 (2002) 22–29.
[5] Y. Narita, T. Tadokoro, T. Ikeda, T. Saiki, S. Mononobe, M. Ohtsu, Near-field raman spectral measurement of polydiacetylene, Appl. Spectrosc. 52 (9) (1998) 1141–1144. [6] Y. Kebukawa, S. Nakashima, K. Nakamura-Messenger, M.E. Zolensky, Submicron distribution of organic matter of carbonaceous chondrite using near-field infrared microspectroscopy, Chem. Lett. 38 (1) (2009) 22–23. [7] Y. Narita, S. Kimura, Fourier transform near-field infrared spectroscopy, Anal. Sci. 17 (2001) i685–i687. [8] M.A. Rahman, T. Oomori, Analysis of protein-induced calcium carbonate crystals in soft coral by near-field IR microspectroscopy, Anal. Sci. 25 (2009) 153–155. [9] M. Hayama, K. Yamamoto, F. Kohori, T. Uesaka, Y. Ueno, H. Sugaya, I. Itagaki, K. Sakai, Nanoscopic behavior of polyvinylpyrrolidone particles on polysulfone/polyvinylpyrrolidone film, Biomaterials 25 (6) (2004) 1019–1028. [10] M. Hayama, K. Yamamoto, F. Kohori, K. Sakai, How polysulfone dialysis membranes containing polyvinylpyrrolidone achieve excellent biocompatibility? J. Membr. Sci. 234 (2004) 41–49. [11] M. Matsuda, K. Yamamoto, T. Yakushiji, M. Fukuda, T. Miyasaka, K. Sakai, Nanotechnological evaluation of protein adsorption on dialysis membrane surface hydrophilized with polyvinylpyrrolidone, J. Membr. Sci. 310 (2008) 219–228. [12] M. Matsuda, H. Sakata, T. Ogawa, K. Yamamoto, T. Yakushiji, M. Fukuda, T. Miyasaka, K. Sakai, Effects of fluid flow on elution of hydrophilic modifier from dialysis membrane surface, J. Artif. Organs 11 (2008) 148–155. [13] L.W. Henderson, C.K. Colton, C.A. Ford, Kinetics of hemofiltration. II. Clinical characterization of a new blood cleansing modality, J. Am. Soc. Nephrol. 8 (3) (1997) 494–508. [14] K. Sakai, Artificial kidney engineering — dialysis membrane and dialyzer for blood purification, J. Chem. Eng. Jpn. 30 (4) (1997) 587–599. [15] J. Barzin, S.S. Madaeni, H. Mirzadeh, M. Mehrabzadeh, Effect of polyvinylpyrrolidone on morphology and performance of hemodialysis membranes prepared from polyether sulfone, J. Appl. Polym. Sci. 92 (6) (2004) 3804–3813. [16] A. Higuchi, K. Shirano, M. Harashima, B.O. Yoon, M. Hara, M. Hattori, K. Imamura, Chemically modified polysulfone hollow fibers with vinylpyrrolidone having improved blood compatibility, Biomaterials 23 (2002) 2659–2666. [17] L. Wan, Z. Xu, Z. Wang, Leaching of PVP from polyacrylonitrile/PVP blending membranes: a comparative study of asymmetric and dense membrane, J. Polym. Sci. Polym. Phys. Ed. 44 (10) (2006) 1490–1498. [18] Q. Yang, T.S. Chung, M. Weber, Microscopic behavior of polyvinylpyrrolidone hydrophilizing agents on phase inversion polyethersulfone hollow fiber membranes for hemofiltration, J. Membr. Sci. 326 (2009) 322–331. [19] L. Rintoul, H. Panayiotou, S. Kokot, G. George, G. Cash, R. Frost, T. Bui, P. Fredericks, Fourier transform infrared spectrometry: a versatile technique for real world samples, Analyst 123 (1998) 571–577. [20] J. Namur, M. Wassef, J.P. Pelage, A. Lewis, M. Manfait, A. Laurent, Infrared microspectroscopy analysis of ibuprofen release from drug eluting beads in uterine tissue, J. Control. Release 135 (2009) 198–202. [21] S.C. Hsu, D. Lin-Vien, R.N. French, Probing the concentration profiles of additives in polymers by IR microspectroscopy: the diffusion of cyasorb UV531 in polypropylene, Appl. Spectrosc. 46 (2) (1992) 225–228. [22] T. Nishioka, N. Teramae, Functional group imaging of a coated interface and polymer blends using Fourier transform infrared microspectroscopy, Anal. Sci. 7 (1991) 1633–1636. [23] J. Kressler, R. Schafer, R. Thomann, Imaging of semicrystalline polymers and polymer blends by FT-IR microspectroscopy, Appl. Spectrosc. 52 (10) (1998) 1269–1273.
Contents lists available at ScienceDirect
Journal of Membrane Science journal homepage: www.elsevier.com/locate/memsci
Functional-group analysis of polyvinylpyrrolidone on the inner surface of hollow-fiber dialysis membranes, by near-field infrared microspectroscopy Sumire Koga a , Taiji Yakushiji b , Masato Matsuda a , Ken-ichiro Yamamoto c , Kiyotaka Sakai a,∗ a b c
Department of Chemical Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan Department of Medical Engineering, Himeji Dokkyo University, 7-2-1 Kamiono, Himeji, Hyogo 670-8524, Japan Waseda Institute for Advanced Study, Waseda University, 1-6-1 Nishiwaseda, Shinjuku-ku, Tokyo 169-8050, Japan
a r t i c l e
i n f o
Article history: Received 28 January 2010 Received in revised form 12 March 2010 Accepted 20 March 2010 Available online 27 March 2010 Keywords: NFIR (near-field infrared microspectroscopy) Dialysis membrane Surface characterization Polyvinylpyrrolidone (PVP)
a b s t r a c t Near-field infrared microspectroscopy (NFIR) is a newly developed surface analysis method that is based on functional-group analysis and has a high spatial resolution. The objective of the present study is to perform nanoscale functional-group analysis of dialysis membrane surfaces by using NFIR. We focused on polyvinylpyrrolidone (PVP), which is employed as an additive to hydrophilize and create pores in synthetic polymer dialysis membranes, and evaluated the PVP distribution on the inner surface of the dialysis membranes. Dialysis membranes made from polysulfone (PSf) and polyester–polymer alloy (PEPA) were first assessed by NFIR and then by attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR). The nanoscale NFIR analysis showed heterogeneous distribution of PVP on the PSf membrane even though the amount of PVP on the membrane surface was high. PVP was homogeneously distributed on the PEPA membrane even though the amount of PVP on the membrane surface was low. In contrast, the microscale ATR-FTIR results showed that PVP was homogeneously distributed on both PSf and PEPA membranes. PVP-rich and PVP-poor regions were distinguishable by NFIR and not by ATR-FTIR, because the spatial resolution of NFIR is higher than that of ATR-FTIR. This study demonstrates for the first time that NFIR can provide nanoscale chemical information on the structures of porous membranes. © 2010 Elsevier B.V. All rights reserved.
1. Introduction In recent times, techniques that employ near-field light (NFL) have attracted much attention. NFL is a localized electromagnetic field that lies close to a material surface. Therefore, unlike propagating light, NFL shows no limitations of spatial resolution resulting from light diffraction. NFL has been used to develop an optical microscope [1] with high spatial resolution and a light record system [2] capable of ultra-density recording. NFL-based techniques are expected to be used for various applications such as Raman spectroscopy and polarization microscopy [3–5]. Near-field infrared microspectroscopy (NFIR) [6–8] is a surface analysis spectroscopic technique that employs NFL. Fig. 1 shows the principle of NFIR. The probe, on approaching the sample, is illuminated and NFL is generated around the probe apex. This NFL is scattered by mutual interactions such as absorption and reflection with the sample. The infrared absorption spectrum is obtained by analyzing the scattered light. The distinctive feature of NFIR is its excellent spatial resolution; nanoscale spot sizes can be obtained using NFIR. In contrast, the spatial resolution of con-
∗ Corresponding author. Tel.: +81 3 5286 3216; fax: +81 3 3209 7957. E-mail address: [email protected] (K. Sakai). 0376-7388/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2010.03.032
ventional infrared spectroscopy is restricted; a spot size below 10 m cannot be obtained with the conventional approach. Therefore, infrared spectroscopy is a microscale analysis method, and this spectroscopic method is incapable of performing functional-group analysis within a 10-m area. However, the spot size obtained by NFIR is the same as the size of the generated NFL which is equal to the radius of curvature of the probe apex; therefore, nanoscale NFL can be obtained by manufacturing a nanoscale probe. Thus, NFIR allows nanoscale analysis of the functional groups. NFIR is a useful analysis technique for obtaining submicroscale to nanoscale chemical information. It also enables the analysis of the functional groups of substances with submicron molecular sizes, which cannot be detected by conventional infrared spectroscopy. Thus, this method can yield chemical information of biomolecules such as proteins. It also enables identification of impurities with submicron sizes and determination of the locations of impurities on sample surfaces. Furthermore, NFIR can elucidate substance distribution by facilitating mapping measurements on 10-m and below areas, which cannot be assessed by conventional infrared spectroscopy. NFIR is a powerful tool for functional-group analysis of nanoscale sample surfaces. Although few studies have attempted to use NFIR for samples that are easy to prepare, such as meteorites [6] and particles [7,8], NFIR for soft synthetic organic materials has not been performed to
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Fig. 2. Chemical structural formula of PSf, PEPA and PVP.
Fig. 1. Principle of near-field infrared microspectoscopy.
date. Because the light spot obtained by NFIR is smaller than that obtained by conventional infrared spectroscopy, the surface roughness and surface thickness hinder the illumination and detection of light, thereby resulting in insufficient light detection. Therefore, samples with coarse surfaces and soft samples that are not sliced are difficult to analyze by NFIR. If such samples can be analyzed by NFIR, this technique can be used for a wide variety of samples. Thus far, we have analyzed dialysis membrane surfaces by both atomic force microscopy (AFM) [9–11] and attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) [12]. Dialysis membranes with adequate diffusive permeability and somewhat biocompatibility are used for the removal of uremic toxins in patients with renal failure [13,14]. Surface analysis [15–18] is essential to evaluate the structure and physicochemical properties of dialysis membrane surfaces, which affect their solute-removal performance and biocompatibility. Nanoscale evaluation is especially important for surface analysis of dialysis membranes. The nanoscale surface structure and physicochemical properties mainly affect the solute-removal performance and biocompatibility of the membrane, because dialysis membranes are nanoscale porous membranes, and mutual interactions at a molecular level cause protein adsorption onto membrane surfaces. However, conventional surface analyses such as AFM and ATR-FTIR cannot provide any nanoscale chemical information on dialysis membrane surfaces. AFM is capable of high-resolution evaluation of the physical structure and physicochemical properties of a material surface. Although AFM affords nanoscale observation of the surface structure, it cannot easily provide chemical information such as chemical composition and functional-group identity. In contrast, ATR-FTIR can be used to analyze the functional groups present on the material surface. However, ATR-FTIR does not provide submicroscale spatial resolution; a light collection system that uses propagating light cannot provide spatial resolution smaller than a wavelength because of the diffraction limit of illuminating light. For example, infrared microspectroscopy [19–23] combined with infrared spectrometry and microscopy provides a spatial resolution of about 10 m.
Therefore, conventional ATR-FTIR cannot provide microscale to nanoscale chemical information of the material surface. Thus, conventional analysis methods such as AFM and ATR-FTIR cannot provide molecule-level information of the chemical composition on the material surface. The objective of the present study is to employ NFIR to perform nanoscale functional-group analysis of dialysis membrane surfaces. We focused on polyvinylpyrrolidone (PVP), which is used as an additive to hydrophilize membrane surfaces and create pores in synthetic polymer dialysis membranes; we evaluated the PVP distribution on the inner surface of dialysis membranes. PVP increases the hydrophilicity of dialysis membranes and plays an important role in their biocompatibility. The membrane is expected to show excellent biocompatibility if PVP shows homogeneous nanoscale distribution on the dialysis membranes. Therefore, it is essential to examine the nanoscale PVP distribution on dialysis membranes. However, such examinations cannot be easily performed using AFM or by elemental analysis using X-ray photoelectron spectroscopy (XPS), because the membrane material is a polymer, and PVP is either mixed with the membrane material or modified on the membrane. Therefore, we used NFIR to analyze PVP distribution on membrane surfaces. Although many synthetic polymer dialysis membranes are commercially available, we used dialysis membranes made from polysulfone (PSf) and polyester–polymer alloy (PEPA), which are representative synthetic polymer dialysis membranes that have been used in Japanese medical institutions. We then tested the same membranes by conventional ATR-FTIR to evaluate the efficacy of NFIR and compared the results of ATR-FTIR with those of NFIR. 2. Materials and methods 2.1. Preparation of membrane samples for ATR-FTIR and NFIR analyses Table 1 shows the technical data on the four commercially available dialysis membranes tested in this study. Fig. 2 shows the structural formulas of PSf, PEPA, and the additive PVP. Four samples were prepared using the dialyzers listed in Table 1 and the following protocol. A hollow-fiber dialysis membrane taken out of the dialyzer was cut to obtain a small fiber sample; then, the central part of the membrane was longitudinally cut open to expose its inner
Table 1 Technical data on the dialysis membranes tested. Sample
Dialyzer
FLX FDX APS0 APSE
FLX-15GW FDX-150GW APS (prototype) APS-15E
Membrane material
Manufacturer
PEPA
Nikkiso
PSf
Asahi Kasei Kuraray Medical
PVP − + − +
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the curvature radius of the probe apex; therefore, nanoscale spatial resolution can be obtained by NFIR. FTIR is used for efficient spectral measurement. The scattered light from mutual interactions between the sample and probe, such as absorption and reflection, was detected. The detected light signal is transformed to a spectrum by Fourier transformation. Although NFL has excellent spatial resolution, the wave number of NFL is the same as that of propagating light. Therefore, the spectrum obtained by NFIR can be analyzed like the spectrum obtained by conventional ATR-FTIR.
Fig. 3. Mapping measurement by ATR-FTIR and NFIR.
surface. The cut edge was pressed by a pincette to fix the opened sample on a sample stage. These prepared test samples were dried at 333 K for 12 h. 2.2. Surface analysis by ATR-FTIR 2.2.1. Spectral measurement of the dialysis membrane surface The spectra of the inner surface of the dialysis membrane were measured by conventional ATR-FTIR. A multi-channel infrared microscope (IRT-7000; JASCO, Tokyo) was attached to an FT-IR Spectrometer (FT/IR-6200; JASCO, Tokyo), and this spectrometer was used as an infrared spectrometer. The spectra were mapped in a 500 m × 1200 m area on the sample surface with measuring points of 40 × 96 (Fig. 3). The number of spectral accumulations was 64 per point. The PVP content ratio for each point was calculated from the obtained spectrum.
2.3.2. Spectral measurement of PVP-coated glass surface PVP (1.0 g; PLASDONE K90; International Specialty Products Inc., New Jersey) was dissolved into 50 ml ethanol to prepare a PVP solution. A cover glass (24 mm × 36 mm; MATSUNAMI, Osaka) was soaked in the PVP solution and then dried at room temperature to prepare a PVP sample. A control sample was prepared without soaking the cover glass in the PVP solution. The spectra of the PVP-coated sample were measured on a near-field infrared microspectroscope (NFIR-200; JASCO, Tokyo). The number of spectral accumulations was 1000 per point. 2.3.3. Spectral measurement of dialysis membrane surface The spectra of the inner surface of the dialysis membrane were measured by NFIR. A near-field infrared microspectroscope (NFIR200; JASCO, Tokyo) equipped with a probe (curvature radius of the probe apex: 990 nm) was used for spectral measurement. The spectra were mapped in a 3 m × 3 m area on sample surface with measuring points of 4 × 4, as shown in Fig. 3. The number of spectral accumulations was 2000 per point. The PVP content ratio for each point was calculated from the obtained spectrum. 3. Results and discussion
2.3. Surface analysis by NFIR
3.1. Surface analysis by ATR-FTIR
2.3.1. Instrumentation Fig. 4 shows a schematic diagram of a near-field infrared microspectroscope (NFIR-200; JASCO, Tokyo). A near-field infrared microspectroscope consists of two systems: a stage system for distance control between the probe and the sample and a spectroscopic system for generation of NFL and detection of scattered light. The stage system uses the same system as AFM and is capable of subnanoscale control of distance and location. In the spectroscopic system, the probe apex (curvature radius of the probe apex: 990 nm) is illuminated by the spectroscopic light to generate NFL around the probe apex. The size of the generated NFL is the same as
3.1.1. Spectral identification Fig. 5 shows the ATR-FTIR spectra of the PEPA and PSf membranes. All the four samples showed a peak at 1580 cm−1 . This peak is derived from a benzene ring present in both PEPA and PSf membranes. Peaks at 1670–1680 cm−1 were obtained for FDX and APSE containing PVP. This peak is derived from the ␥-lactam present in PVP. 3.1.2. Microscale analysis of the dialysis membrane surface From the spectral data, the ratio of the peak areas of PVP and the polymer membranes was calculated to obtain a PVP content ratio, and two-dimensional images of the PVP content ratio were obtained (Fig. 6). The magnitude of the PVP content ratio was depicted using different colors depending on the PVP content ratio indicated in the right-hand side vertical scale. We evaluated the PVP distribution by using the PVP content ratio instead of the PVP peak area to eliminate sample errors ascribable to differences in the adhesion of the ATR-FTIR prism with the membrane surface. The presence of an air layer between the ATR-FTIR prism and membrane surface lowers the PVP peak area. The PVP content ratio of FLX and APS0 was 0. Although the PVP content ratio of FDX was lower than that of APSE, PVP was distributed homogeneously in both samples. 3.2. Surface analysis by NFIR
Fig. 4. Schematic diagram of near-field infrared microspectroscope.
3.2.1. Spectral analysis of PVP-coated glass surface Fig. 7 shows the spectra of the PVP-coated glass surface and untreated glass surface (control sample). The obtained spectra were treated by Kramers–Kronig (KK) transformation to convert transmittance into a complex refraction index k. In the case of NFIR,
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Fig. 7. PVP-coated glass surface spectra by NFIR.
Fig. 5. Dialysis membrane surface spectra by ATR-FTIR.
the probe apex was exposed to light to generate NFL around the apex. When the sample approaches the probe apex, the NFL is scattered via mutual interactions between the sample and the probe apex. Because NFIR detects the scattered light, the obtained spectra are essentially reflection spectra. The KK transformation is a method for analyzing reflection spectra and arithmetically comparing reflection spectra with transmission and absorption spectra. Peaks at 1671 cm−1 and 1243 cm−1 were observed for the PVPcoated glass surface (Fig. 7). A peak at 1228 cm−1 was observed for the glass surface. Peaks ranging from 1228 cm−1 to 1243 cm−1 , which were observed for both samples, were derived from the glass, and a peak at 1671 cm−1 was derived from the PVP. These results indicate that the wave number of the PVP peak observed in NFIR is almost similar to that of the PVP peak observed in ATR-FTIR.
Fig. 8. Dialysis membrane surface spectra by NFIR.
3.2.2. Nanoscale analysis of the dialysis membrane surface Fig. 8 shows the spectra of the PEPA and PSf membranes. The spectral data were transformed by the KK transformation from transmittance into the complex refraction index k. Peaks derived from PVP were observed at around 1670 cm−1 for both FDX and APSE. All four samples showed peaks at around
Fig. 6. PVP content ratio of dialysis membrane surface by ATR-FTIR (two-dimensional distribution).
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Fig. 9. PVP content ratio of dialysis membrane surface by NFIR (two-dimensional distribution).
1580 cm−1 . The latter peaks are derived from a benzene ring present in both PEPA and PSf membranes. The spectral data were used to calculate the peak area ratios of PVP and the polymer membrane and determine the PVP content ratio, and two-dimensional images of the PVP content ratio were obtained (Fig. 9). The magnitude of the PVP content ratio was depicted using different colors depending on the PVP content ratio indicated in the righthand side vertical scale. We evaluated the PVP distribution using the PVP content ratio instead of the PVP peak area to eliminate sample errors caused by differences in membrane surface roughness, which lowers the PVP peak area. The PVP content ratio of FLX and APS0 was 0. FDX and APSE showed heterogeneous PVP distribution, with the PVP-rich and PVP-poor regions forming a mosaic. Fig. 10 shows the average, maximum, and minimum values for the PVP content ratios of FDX and APSE. The PVP content ratio for APSE was high but the overall PVP content ratio for FDX was low, and the membrane had a few regions with no PVP. In our previous study, in which the adsorbability of human serum albumin (HSA) on the inner surface of a dialysis membrane [11] was measured by AFM, a local region showing high HSA adsorbability was observed in FDX. These results indicate the presence of some regions with no PVP, which corresponds to the results obtained by NFIR. Standard deviation indicates that the dispersion of the PVP content ratio for APSE is higher than that for FDX. Thus, although the quantity of PVP on the membrane surface of APSE is higher than that on the membrane surface of FDX, PVP homogeneity for APSE is lower than that for FDX. NFIR measurement is a nanoscale functional-group analysis because the spot size is 990 nm. These results demonstrate that NFIR can be used to obtain nanoscale chemical information on dialysis membrane surfaces.
3.3. Comparison of the measurement results obtained by ATR-FTIR with those obtained by NFIR Fig. 11 shows the average, maximum, and minimum ATR-FTIR and NFIR values for the PVP content ratios of FDX and APSE. The NFIR results are valid by the lack of significant differences in the average PVP content ratios obtained using conventional ATR-FITR and NFIR. However, the difference between the maximum and minimum PVP content ratios obtained using ATR-FTIR and NFIR was different from the corresponding value obtained using NFIR; the difference for NFIR was higher than that for ATR-FTIR. These results indicate that the PVP-rich and PVP-poor regions are distinguish-
Fig. 10. PVP content ratio of dialysis membrane surface by NFIR (average, maximum, minimum).
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Fig. 11. Comparison of PVP content ratio results by ATR-FTIR with those by NFIR (average, maximum, minimum) (n = 80: 16 measuring points × 5 times).
able by NFIR, because the spatial resolution of NFIR is much higher than that of ATR-FTIR. In the conventional microscale functionalgroup analysis by ATR-FTIR, each dialysis membrane was found to be homogeneously covered by PVP particles, and there was no difference in the PVP distributions of the PEPA and PSf membranes. However, nanoscale functional-group analysis by NFIR, which is capable of analyzing an area smaller than the spot size obtained by ATR-FTIR, demonstrates that PVP is heterogeneously distributed on dialysis membranes, and PVP homogeneity for the PEPA membrane is higher than that for the PSf membrane. In addition, NFIR analysis revealed regions on the PEPA membrane where the membrane material was partially exposed due to a lack of PVP on the PEPA membrane. Therefore, surface analysis by NFIR is effective for exact evaluation of PVP distribution on dialysis membrane surfaces. 4. Conclusions Although conventional microscale measurement shows homogeneous distribution of PVP on the surface of dialysis membranes, nanoscale analysis by NFIR shows heterogeneous distribution of PVP on the membrane surface and differences in the PVP homogeneities of different dialysis membranes. In addition, NFIR reveals regions where the membrane material is partially exposed due to a lack of PVP on the PEPA membrane. The present study demonstrates for the first time that NFIR can be used for nanoscale assessments of the structure of porous membranes. Acknowledgment This study was partly supported by the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. References [1] V.F. Dryakhlushin, V.P. Veiko, N.B. Voznesenskii, Scanning near-field optical microscopy and near-field optical probes: properties, fabrication, and control of parameters, Quantum Electron. 37 (2) (2007) 193–203. [2] K. Kurihara, Y. Yamakawa, T. Shima, J. Tominaga, Capacity increase in radical direction of super-resolution near-field structure read-only-memory disc, Jpn. J. Appl. Phys. 43 (6B) (2007) 3898–3901. [3] T. Inoue, F. Sato, Y. Narita, Near-field fiber probe for polarization spectroscopy, Vib. Spectrosc. 35 (2004) 33–37. [4] T. Saiki, Y. Narita, Nano-optical imaging spectroscopy: recent advances in nearfield scanning optical microscopy, JSAP Int. 5 (2002) 22–29.
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