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Effect of plasma treatment on surface chemical-bonding states and electrical properties of polyacrylonitrile nanofibers
Thin Solid Films 519 (2011) 7090–7094
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Effect of plasma treatment on surface chemical-bonding states and electrical properties of polyacrylonitrile nanofibers Y.H. Kang a, K. Ahn a, S.Y. Jeong a, J.S. Bae b, J.S. Jin b, H.G. Kim b, S.W. Hong a, C.R. Cho a,⁎ a b
College of Nanoscience and Nanotechnology, Pusan National University, Busan 609-735, Republic of Korea Busan Center, Korea Basic Science Institute, Busan 609-735, Republic of Korea
a r t i c l e
i n f o
Available online 22 April 2011 Keywords: Plasma treatment Polyacrylonitrile Nanofiber Hydrophilicity Biosensor Surface energy
a b s t r a c t Electrospun polyacrylonitrile (PAN) nanofibers were subjected to surface modification by atmospheric pressure (AP) plasma treatment with reactive gases. There was no damage to the surfaces after this plasma treatment, and no significant changes were observed in the morphologies of the nanofibers. The surface energies of O2- and N2plasma-treated PAN (abbreviated as OPP and NPP, respectively) nanofibers increased by almost 138.7% and 190.6%, respectively, in comparison with that of an untreated nanofiber (256.6 mJ/m2). The binding energies of both OPP and NPP samples increased through the formation of many hydrophilic bonds involving oxygen. The current–voltage (I–V) characteristics of the nanofibers were determined for the different reactive gases, and the plasma-treated nanofibers showed higher protein immobilization compared to the untreated ones. This result indicates that electrospun PAN nanofibers have the potential to be used in protein biosensor systems. © 2011 Elsevier B.V. All rights reserved.
1. Introduction As protein biosensor systems and technologies become more advanced, attention is being focused on the immobilization of enzymes. Such enzyme immobilization strongly depends on substrate properties such as surface area and hydrophilicity [1,2]. Nanostructured supports are believed to have the capacity to ensure the activity and immobilization efficiency of biomolecules [3]. Recently, there has been a lot of interest in one-dimensional (1D) nanostructures because of several desirable properties such as their large surface areas, improved mechanical properties, and flexibility in surface functionalities. Compared with other nanostructures, nanofibers, which have the advantages of high porosity and interconnectivity, are applied in various fields such as tissue engineering, biosensors, and filter systems [4–7]. Among the processes available for nanofiber fabrication, the electrospinning technique is a simple and versatile method that uses electrostatic forces to produce nanofibrous structures with large surface areas [8]. Synthetic polymers such as poly(ε-caprolactone) (PCL), polyacrylonitrile (PAN), poly(vinyl alcohol) (PVA), and poly(methyl methacrylate) (PMMA) are the most commonly used materials for the preparation of nanofibers [3,8]. As one of the abovementioned polymers, PAN is a commercially important compound that has the advantages of easy transformation to a fiber-type shape, high flexibility in structure control, and good stability in different environments [8,9]. PAN has been studied for biological applications in many different fields [1,10–12] because of its hydrophilicity, which is due to the presence of active C`N groups.
⁎ Corresponding author. Tel.: + 82 55 350 5297; fax: + 82 55 353 1314. E-mail address: [email protected] (C.R. Cho). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.04.056
However, the hydrophilicity of pure PAN is insufficient for its attachment to and reaction with biomolecules [13]. The surface modification can be achieved by various techniques, including discharge, plasma, photon, electron-beam, ion-beam, and X-ray methods [14,15]. In the present study, atmospheric plasma (AP) treatment was used to modify the surfaces of electrospun PAN nanofibers. AP treatment is a powerful method that can be used to improve the surface characteristics of a material without affecting its bulk properties [16–18]. Such treatment can usually induce active sites where grafting occurs, and as a result, new functional monomers may be grafted onto the polymer surface [19–22]. Some researchers have studied plasma treatment with reactive gases for enhancing the hydrophilicity and permeability of PAN membranes [23–26]. In this study, we present the effect of AP treatment on the surface and electrical properties of PAN nanofibers. We fabricated the PAN nanofibers on Pt-patterned silicon, and the surfaces of the nanofibers were modified by AP treatment with the reactive gases H2, O2, and N2. The resulting changes in the surface energy, chemical-bonding states, and functional groups on the surface of the AP-treated PAN nanofibers were measured. The changes in the resistance of the AP-treated PAN nanofibers were also examined. 2. Experimental details A 16.5 wt.% PAN solution was prepared by dissolving PAN in N,Ndimethylformamide (DMF) and stirring the solution for 24 h at 80 °C. This solution was used to fabricate nanofibers through the electrospinning technique. A voltage of 14.5 kV was applied between the tip of the needle and the Pt electrodes on the aluminum collector. The PAN solution was injected with a feed rate of 0.25 mL/h into a 10-mL
Y.H. Kang et al. / Thin Solid Films 519 (2011) 7090–7094
syringe with a capillary tip; the syringe had an inner diameter of 0.51 mm. Subsequently, the solution was electrospun directly on the Pt interdigital electrodes (Pt-IDEs) [27]. The AP plasma system [FemtoScience, Plasmaflux™], consisting of a 13.56-MHz radiofrequency (RF) generator, an RF plasma reactor, and a moving stage, was used in this study. Argon was used as the carrier gas, and H2, O2, and N2 were used as the reactive gases. The distance between the sample and the plasma of the reactor was 0.5 cm, and the PAN nanofibers on the Pt-IDEs were treated with H2/Ar (flow ratio: 0.4 and power: 36 W), O2/Ar (flow ratio: 0.01 and power: 35 W), and N2/Ar (flow ratio: 0.01 and power: 11 W) for 10 min. The fabricated nanofibrous substrates were observed using a field-emission scanning electron microscope (SEM) [Hitachi, S-4700]. The wettability of the PAN nanofibers was measured by contact-angle (CA) measurements [Dataphysics, OCA10], and their surface energy was calculated using the extended Fowkes equation as follows [28]: qffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffi σL ð1 + cos θÞ = 2 σLD σSD + σLP σSP + σLH σSH
ð1Þ
where σL and σS are the surface energies of the test liquid and the sample, respectively. The superscripts D, P, and H refer to the dispersive, polar, and hydrogen-bonding (H\H) components of the surface energy, respectively. The three components σSD, σSP, and σSH can be calculated using Eq. (1), and the total surface energy of the solid sample (σS) can be obtained from Eq. (2): D
P
H
σS = σS + σS + σS :
ð2Þ
In this study, deionized water, glycerol, and ethylene glycol were used for the measurements. The surface-energy values of each liquid are reported elsewhere [29]. The surface chemical functional groups of the PAN nanofibers were investigated by Fourier transform infrared (FTIR) spectroscopy [JASCO, FT/IR 6300] in the range 4000–400 cm− 1 at a resolution of
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4 cm− 1. The chemical-bonding states and atomic content ratio of the PAN nanofibers were examined by X-ray photoelectron spectroscopy (XPS) [VG Scientific, ESCALAB250] using a hemispherical electrostatic energy analyzer and an AlKα (1486.6 eV) X-ray source. The C1s peak was deconvoluted using Gaussian curves as fitting functions. The electrical properties of the samples were determined by using a semiconductor device [MS-TECH, MST-4000A], and the changes in their resistance were examined for the different reactive gases. 3. Results and discussion Fig. 1 shows SEM images of the untreated and plasma-treated PAN (PP) nanofibers. The diameter of the nanofibers was constant at about 800 nm, and there were no beads. There was no significant damage to the nanofiber surfaces, and there was no change in the apparent density of the nanofibers after plasma treatment with H2, O2, and N2 gases. Fig. 2 shows the wettabilities and surface energies of the samples, as obtained by CA measurements performed using water, glycerol, and ethylene glycol. While the water and glycerol droplets spread out to a greater extent on the PAN nanofibers subjected to O2- and N2plasma treatment, the CAs increased in the case of H2-plasma-treated PAN (HPP) nanofibers. The ethylene glycol droplets spread out over all the nanofibers (Fig. 2a). This result is attributed to redeposition and to the etching effect of the reactive ions [20]. The surface energy was calculated with the extended Fowkes equation using the CA results [28,29]. Fig. 2b shows the total surface energy and its three components (dispersive, polar, and H\H components) for the PAN nanofibers. The changes in the surface energies of all samples were observed without any notable morphological change on the surfaces of the AP-treated PAN nanofibers [30–32]. The surface energies of O2and N2-plasma-treated PAN (abbreviated as OPP and NPP, respectively) nanofibers increased significantly, because of the increase in the number of intermolecular polar bonds due to the generation of hydrophilic functional groups on the surface of the nanofibers. The total surface energies (SEs) of the OPP and NPP nanofibers increased
Fig. 1. SEM images of (a) an untreated PAN nanofiber, and (b–d) PAN nanofibers treated by AP plasma with H2, O2, and N2 reactive gases, respectively.
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Fig. 3. FTIR spectra of (a) an untreated PAN nanofiber, and (b–d) PAN nanofibers treated by AP plasma with H2, O2, and N2 reactive gases, respectively (ν is stretching vibration and δ is bending vibration).
Fig. 2. (a) Contact angles and (b) surface energies of an untreated nanofiber and PAN nanofibers treated by AP plasma with reactive gases (■: total surface energy, ○: dispersive component, ⋄: polar component and ▽: H\H component).
by almost 139% and 191%, respectively, in comparison to that of the untreated nanofibers (256.6 mJ/m2). However, the SE of the HPP nanofibers decreased to 104.53 mJ/m2. This is considered to be caused by a significant decrease in the dispersive and polar components and an increase in the H\H component. These observations imply that new chemical bonds formed on the surface of nanofibers subjected to plasma treatment lead to changes in the wettability and SE [20,21,26]. The chemical functional species on PAN nanofibers include carbon chains, the nitrile group (C`N), and methylene groups (CH\ and CH2\), which correspond to peaks at 1100 (C\C), 2240 (C`N), 2930 (CH2 stretching vibration), and 1450 cm− 1 (CH2 bending vibration) in the FTIR spectra (Fig. 3a) [33,34]. However, a gradual decrease is observed at 2240 cm− 1, which results from the conversion of nitrile bonds to imine bonds (C_N), and implies that plasma treatment can break the C`N bond. Further, the peaks at 2930, 1450, and 1100 cm− 1 decrease after the treatment, and the peak at 1540 cm− 1 (related to the \NH bond) is mainly due to hydrogen atoms in broken triple bonds. PP nanofibers show the presence of the C_O group (peak at 1650 cm− 1) and the OH stretching vibration mode in the range 3500–4000 cm− 1, as shown in Fig. 3b–d. It is thought that the hydrophilic functional groups containing the OH group at the surface could be induced by plasma modification, especially by O2- and N2-plasma treatments [20,33]. The increase in the intensities of the peaks in the range 3400–4000 cm− 1 after H2-plasma treatment occurs as a result of the broken nitrile groups combining with oxygen and water molecules in air during the AP process. The intensity of the 1650 cm− 1 peak increases significantly in the spectra of the HPP nanofibers (Fig. 3b), whereas no significant increase is observed with the O2- and N2-plasma treatments. This result
may be due to the escape of a larger amount of oxygen in the form of CO and CO2 gases in the samples subjected to O2- and N2-plasma treatments compared to the amount of oxygen escaping in the case of H2-plasma treatment. The plasma process produces a large number of unsaturated bonds and a large number of free radicals on the nanofiber surfaces; the free radicals can react with other atoms to form new functional groups, and it is known that the SE of the samples changes significantly because of this [19–21]. To investigate the generation of new chemical groups on the PP nanofibers by the plasma process, we analyzed the chemical-bonding states of the samples using XPS. The results of the chemical composition analysis gave N/C ratios of untreated, HPP, OPP, and NPP nanofibers of 0.24, 0.08, 0.27, and 0.34, respectively. Fig. 4 shows the deconvoluted C1s peaks for untreated PAN and PP nanofibers. The spectrum of the untreated PAN shows the three surface-energy components to be 284.7 (54.9%), 285.8 (44.1%), and 288.5 (1.0%) eV, which correspond to \CH2 and \CH, \CN, and \COOR, respectively (Fig. 4a) [23,26]. After plasma treatment, additional species appear on the surface of the nanofibers. In the case of HPP nanofibers, the amide (\CONH2) and oxygen (C\O) functional group peaks at 286.5 eV appeared simultaneously with a decrease in the intensity of the \CN peak. Because of the characteristics of the atmospheric plasma, oxygen-related groups could also be generated on the NPP and HPP nanofibers. As the contribution of the methylene group to the nonpolar surface-energy components increased, the peaks of all components shifted in such a way that they corresponded to lower binding energies. The C1s spectrum of the OPP and NPP samples was deconvoluted to six peaks because of the formation of new species, i.e., amide (\CONH2) and carbon–oxygen bonds (C\O, C_O), carboxyl (\COOH), and carbonyl (\OC_O) groups, respectively (Fig. 4c and d). The C1s peak of the OPP and NPP samples shifted to a high binding energy because of the formation of many hydrophilic bonds, which increased the SE. This behavior is similar to that found from the CA measurements. Fig. 5 shows the resistances of the PAN nanofibers prepared on PtIDEs (inset in Fig. 5) measured at different applied voltages for the different plasma gases. The resistances of the untreated, HPP, OPP, and NPP samples at 3 V were 3.2 × 1011, 7.6 × 1011, 3.1 × 1011, and 4.2 × 1010 Ω, respectively. Compared to the resistance of the untreated samples, that of the HPP samples was found to increase by 137.5%, and that of the NPP ones to decrease by 86.9%, whereas no meaningful change was found in the OPP samples. It has previously been reported that the increase in the electrical conductivity of a polymeric surface is mainly caused by the incorporated nitrogen and the conjugation of
Y.H. Kang et al. / Thin Solid Films 519 (2011) 7090–7094
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Fig. 5. Variation in resistance according to the applied voltage for the untreated PAN nanofiber and the PAN nanofibers treated by AP plasma with H2, O2, and N2 reactive gases. The inset shows the patterned Pt interdigital electrodes (Pt-IDEs) on the silicon substrate. The distance between electrode fingers was 10 μm.
4. Conclusion In this study, we have indicated that plasma surface treatment causes two changes in the SE and resistance of PAN nanofibers, without any changes in surface morphology. The increase of the polar component in the total SE promotes the formation of hydrophilic bonds, thus enhancing the surface wettability of AP-treated PAN nanofibers. From the FTIR and XPS results, it is observed that many new functional groups are produced on the surfaces of AP-treated PAN nanofibers with different reactive gases, and as a result, changes in the SE and wettability are obtained. Also, the regenerated and redeposited functional groups, which are induced by AP treatment, could affect the resistance of PAN nanofibers. After N2-plasma treatment of a PAN nanofiber, not only was an improvement in SE and wettability observed, but also a change in the electrical properties, caused by the new functional groups. Therefore, it can be expected that applications may be found in the immobilization and detection of biomolecules in electrical biosensor systems. Acknowledgment This study was financially supported by the Korea Research Foundation Grant funded by the Korean Government (KRF-2008313-D00607). Fig. 4. XPS C1s high-resolution spectra of (a) an untreated PAN nanofiber, and (b–d) PAN nanofibers treated by AP plasma with H2, O2, and N2 reactive gases, respectively (the C1s peak is deconvoluted to several peaks).
carbon double bonds [35]. We expected that the N2-plasma treatment would increase the conductivity of the PAN nanofibers by inducing the formation of many types of nitrogen bonds on the PAN surface; this is supported by the chemical composition of nitrogen in the XPS results. Although there is a difference in the resistance of about one order of magnitude between the untreated and N2-plasma-treated PAN nanofibers, this is sufficient to distinguish the change in electrical properties. Thus, these plasma-treated nanofibers could find applications in biosensors used for detecting biomaterials that become attached to them when the resistance changes. From the results of this study, we believe that the effect of plasma treatment on a PAN-based material with high conductivity will be larger than on one with low conductivity because of the rapid transfer of charge generated by the plasma through the bulk nanofiber in the former material.
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Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Effect of plasma treatment on surface chemical-bonding states and electrical properties of polyacrylonitrile nanofibers Y.H. Kang a, K. Ahn a, S.Y. Jeong a, J.S. Bae b, J.S. Jin b, H.G. Kim b, S.W. Hong a, C.R. Cho a,⁎ a b
College of Nanoscience and Nanotechnology, Pusan National University, Busan 609-735, Republic of Korea Busan Center, Korea Basic Science Institute, Busan 609-735, Republic of Korea
a r t i c l e
i n f o
Available online 22 April 2011 Keywords: Plasma treatment Polyacrylonitrile Nanofiber Hydrophilicity Biosensor Surface energy
a b s t r a c t Electrospun polyacrylonitrile (PAN) nanofibers were subjected to surface modification by atmospheric pressure (AP) plasma treatment with reactive gases. There was no damage to the surfaces after this plasma treatment, and no significant changes were observed in the morphologies of the nanofibers. The surface energies of O2- and N2plasma-treated PAN (abbreviated as OPP and NPP, respectively) nanofibers increased by almost 138.7% and 190.6%, respectively, in comparison with that of an untreated nanofiber (256.6 mJ/m2). The binding energies of both OPP and NPP samples increased through the formation of many hydrophilic bonds involving oxygen. The current–voltage (I–V) characteristics of the nanofibers were determined for the different reactive gases, and the plasma-treated nanofibers showed higher protein immobilization compared to the untreated ones. This result indicates that electrospun PAN nanofibers have the potential to be used in protein biosensor systems. © 2011 Elsevier B.V. All rights reserved.
1. Introduction As protein biosensor systems and technologies become more advanced, attention is being focused on the immobilization of enzymes. Such enzyme immobilization strongly depends on substrate properties such as surface area and hydrophilicity [1,2]. Nanostructured supports are believed to have the capacity to ensure the activity and immobilization efficiency of biomolecules [3]. Recently, there has been a lot of interest in one-dimensional (1D) nanostructures because of several desirable properties such as their large surface areas, improved mechanical properties, and flexibility in surface functionalities. Compared with other nanostructures, nanofibers, which have the advantages of high porosity and interconnectivity, are applied in various fields such as tissue engineering, biosensors, and filter systems [4–7]. Among the processes available for nanofiber fabrication, the electrospinning technique is a simple and versatile method that uses electrostatic forces to produce nanofibrous structures with large surface areas [8]. Synthetic polymers such as poly(ε-caprolactone) (PCL), polyacrylonitrile (PAN), poly(vinyl alcohol) (PVA), and poly(methyl methacrylate) (PMMA) are the most commonly used materials for the preparation of nanofibers [3,8]. As one of the abovementioned polymers, PAN is a commercially important compound that has the advantages of easy transformation to a fiber-type shape, high flexibility in structure control, and good stability in different environments [8,9]. PAN has been studied for biological applications in many different fields [1,10–12] because of its hydrophilicity, which is due to the presence of active C`N groups.
⁎ Corresponding author. Tel.: + 82 55 350 5297; fax: + 82 55 353 1314. E-mail address: [email protected] (C.R. Cho). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.04.056
However, the hydrophilicity of pure PAN is insufficient for its attachment to and reaction with biomolecules [13]. The surface modification can be achieved by various techniques, including discharge, plasma, photon, electron-beam, ion-beam, and X-ray methods [14,15]. In the present study, atmospheric plasma (AP) treatment was used to modify the surfaces of electrospun PAN nanofibers. AP treatment is a powerful method that can be used to improve the surface characteristics of a material without affecting its bulk properties [16–18]. Such treatment can usually induce active sites where grafting occurs, and as a result, new functional monomers may be grafted onto the polymer surface [19–22]. Some researchers have studied plasma treatment with reactive gases for enhancing the hydrophilicity and permeability of PAN membranes [23–26]. In this study, we present the effect of AP treatment on the surface and electrical properties of PAN nanofibers. We fabricated the PAN nanofibers on Pt-patterned silicon, and the surfaces of the nanofibers were modified by AP treatment with the reactive gases H2, O2, and N2. The resulting changes in the surface energy, chemical-bonding states, and functional groups on the surface of the AP-treated PAN nanofibers were measured. The changes in the resistance of the AP-treated PAN nanofibers were also examined. 2. Experimental details A 16.5 wt.% PAN solution was prepared by dissolving PAN in N,Ndimethylformamide (DMF) and stirring the solution for 24 h at 80 °C. This solution was used to fabricate nanofibers through the electrospinning technique. A voltage of 14.5 kV was applied between the tip of the needle and the Pt electrodes on the aluminum collector. The PAN solution was injected with a feed rate of 0.25 mL/h into a 10-mL
Y.H. Kang et al. / Thin Solid Films 519 (2011) 7090–7094
syringe with a capillary tip; the syringe had an inner diameter of 0.51 mm. Subsequently, the solution was electrospun directly on the Pt interdigital electrodes (Pt-IDEs) [27]. The AP plasma system [FemtoScience, Plasmaflux™], consisting of a 13.56-MHz radiofrequency (RF) generator, an RF plasma reactor, and a moving stage, was used in this study. Argon was used as the carrier gas, and H2, O2, and N2 were used as the reactive gases. The distance between the sample and the plasma of the reactor was 0.5 cm, and the PAN nanofibers on the Pt-IDEs were treated with H2/Ar (flow ratio: 0.4 and power: 36 W), O2/Ar (flow ratio: 0.01 and power: 35 W), and N2/Ar (flow ratio: 0.01 and power: 11 W) for 10 min. The fabricated nanofibrous substrates were observed using a field-emission scanning electron microscope (SEM) [Hitachi, S-4700]. The wettability of the PAN nanofibers was measured by contact-angle (CA) measurements [Dataphysics, OCA10], and their surface energy was calculated using the extended Fowkes equation as follows [28]: qffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffiffi σL ð1 + cos θÞ = 2 σLD σSD + σLP σSP + σLH σSH
ð1Þ
where σL and σS are the surface energies of the test liquid and the sample, respectively. The superscripts D, P, and H refer to the dispersive, polar, and hydrogen-bonding (H\H) components of the surface energy, respectively. The three components σSD, σSP, and σSH can be calculated using Eq. (1), and the total surface energy of the solid sample (σS) can be obtained from Eq. (2): D
P
H
σS = σS + σS + σS :
ð2Þ
In this study, deionized water, glycerol, and ethylene glycol were used for the measurements. The surface-energy values of each liquid are reported elsewhere [29]. The surface chemical functional groups of the PAN nanofibers were investigated by Fourier transform infrared (FTIR) spectroscopy [JASCO, FT/IR 6300] in the range 4000–400 cm− 1 at a resolution of
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4 cm− 1. The chemical-bonding states and atomic content ratio of the PAN nanofibers were examined by X-ray photoelectron spectroscopy (XPS) [VG Scientific, ESCALAB250] using a hemispherical electrostatic energy analyzer and an AlKα (1486.6 eV) X-ray source. The C1s peak was deconvoluted using Gaussian curves as fitting functions. The electrical properties of the samples were determined by using a semiconductor device [MS-TECH, MST-4000A], and the changes in their resistance were examined for the different reactive gases. 3. Results and discussion Fig. 1 shows SEM images of the untreated and plasma-treated PAN (PP) nanofibers. The diameter of the nanofibers was constant at about 800 nm, and there were no beads. There was no significant damage to the nanofiber surfaces, and there was no change in the apparent density of the nanofibers after plasma treatment with H2, O2, and N2 gases. Fig. 2 shows the wettabilities and surface energies of the samples, as obtained by CA measurements performed using water, glycerol, and ethylene glycol. While the water and glycerol droplets spread out to a greater extent on the PAN nanofibers subjected to O2- and N2plasma treatment, the CAs increased in the case of H2-plasma-treated PAN (HPP) nanofibers. The ethylene glycol droplets spread out over all the nanofibers (Fig. 2a). This result is attributed to redeposition and to the etching effect of the reactive ions [20]. The surface energy was calculated with the extended Fowkes equation using the CA results [28,29]. Fig. 2b shows the total surface energy and its three components (dispersive, polar, and H\H components) for the PAN nanofibers. The changes in the surface energies of all samples were observed without any notable morphological change on the surfaces of the AP-treated PAN nanofibers [30–32]. The surface energies of O2and N2-plasma-treated PAN (abbreviated as OPP and NPP, respectively) nanofibers increased significantly, because of the increase in the number of intermolecular polar bonds due to the generation of hydrophilic functional groups on the surface of the nanofibers. The total surface energies (SEs) of the OPP and NPP nanofibers increased
Fig. 1. SEM images of (a) an untreated PAN nanofiber, and (b–d) PAN nanofibers treated by AP plasma with H2, O2, and N2 reactive gases, respectively.
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Fig. 3. FTIR spectra of (a) an untreated PAN nanofiber, and (b–d) PAN nanofibers treated by AP plasma with H2, O2, and N2 reactive gases, respectively (ν is stretching vibration and δ is bending vibration).
Fig. 2. (a) Contact angles and (b) surface energies of an untreated nanofiber and PAN nanofibers treated by AP plasma with reactive gases (■: total surface energy, ○: dispersive component, ⋄: polar component and ▽: H\H component).
by almost 139% and 191%, respectively, in comparison to that of the untreated nanofibers (256.6 mJ/m2). However, the SE of the HPP nanofibers decreased to 104.53 mJ/m2. This is considered to be caused by a significant decrease in the dispersive and polar components and an increase in the H\H component. These observations imply that new chemical bonds formed on the surface of nanofibers subjected to plasma treatment lead to changes in the wettability and SE [20,21,26]. The chemical functional species on PAN nanofibers include carbon chains, the nitrile group (C`N), and methylene groups (CH\ and CH2\), which correspond to peaks at 1100 (C\C), 2240 (C`N), 2930 (CH2 stretching vibration), and 1450 cm− 1 (CH2 bending vibration) in the FTIR spectra (Fig. 3a) [33,34]. However, a gradual decrease is observed at 2240 cm− 1, which results from the conversion of nitrile bonds to imine bonds (C_N), and implies that plasma treatment can break the C`N bond. Further, the peaks at 2930, 1450, and 1100 cm− 1 decrease after the treatment, and the peak at 1540 cm− 1 (related to the \NH bond) is mainly due to hydrogen atoms in broken triple bonds. PP nanofibers show the presence of the C_O group (peak at 1650 cm− 1) and the OH stretching vibration mode in the range 3500–4000 cm− 1, as shown in Fig. 3b–d. It is thought that the hydrophilic functional groups containing the OH group at the surface could be induced by plasma modification, especially by O2- and N2-plasma treatments [20,33]. The increase in the intensities of the peaks in the range 3400–4000 cm− 1 after H2-plasma treatment occurs as a result of the broken nitrile groups combining with oxygen and water molecules in air during the AP process. The intensity of the 1650 cm− 1 peak increases significantly in the spectra of the HPP nanofibers (Fig. 3b), whereas no significant increase is observed with the O2- and N2-plasma treatments. This result
may be due to the escape of a larger amount of oxygen in the form of CO and CO2 gases in the samples subjected to O2- and N2-plasma treatments compared to the amount of oxygen escaping in the case of H2-plasma treatment. The plasma process produces a large number of unsaturated bonds and a large number of free radicals on the nanofiber surfaces; the free radicals can react with other atoms to form new functional groups, and it is known that the SE of the samples changes significantly because of this [19–21]. To investigate the generation of new chemical groups on the PP nanofibers by the plasma process, we analyzed the chemical-bonding states of the samples using XPS. The results of the chemical composition analysis gave N/C ratios of untreated, HPP, OPP, and NPP nanofibers of 0.24, 0.08, 0.27, and 0.34, respectively. Fig. 4 shows the deconvoluted C1s peaks for untreated PAN and PP nanofibers. The spectrum of the untreated PAN shows the three surface-energy components to be 284.7 (54.9%), 285.8 (44.1%), and 288.5 (1.0%) eV, which correspond to \CH2 and \CH, \CN, and \COOR, respectively (Fig. 4a) [23,26]. After plasma treatment, additional species appear on the surface of the nanofibers. In the case of HPP nanofibers, the amide (\CONH2) and oxygen (C\O) functional group peaks at 286.5 eV appeared simultaneously with a decrease in the intensity of the \CN peak. Because of the characteristics of the atmospheric plasma, oxygen-related groups could also be generated on the NPP and HPP nanofibers. As the contribution of the methylene group to the nonpolar surface-energy components increased, the peaks of all components shifted in such a way that they corresponded to lower binding energies. The C1s spectrum of the OPP and NPP samples was deconvoluted to six peaks because of the formation of new species, i.e., amide (\CONH2) and carbon–oxygen bonds (C\O, C_O), carboxyl (\COOH), and carbonyl (\OC_O) groups, respectively (Fig. 4c and d). The C1s peak of the OPP and NPP samples shifted to a high binding energy because of the formation of many hydrophilic bonds, which increased the SE. This behavior is similar to that found from the CA measurements. Fig. 5 shows the resistances of the PAN nanofibers prepared on PtIDEs (inset in Fig. 5) measured at different applied voltages for the different plasma gases. The resistances of the untreated, HPP, OPP, and NPP samples at 3 V were 3.2 × 1011, 7.6 × 1011, 3.1 × 1011, and 4.2 × 1010 Ω, respectively. Compared to the resistance of the untreated samples, that of the HPP samples was found to increase by 137.5%, and that of the NPP ones to decrease by 86.9%, whereas no meaningful change was found in the OPP samples. It has previously been reported that the increase in the electrical conductivity of a polymeric surface is mainly caused by the incorporated nitrogen and the conjugation of
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Fig. 5. Variation in resistance according to the applied voltage for the untreated PAN nanofiber and the PAN nanofibers treated by AP plasma with H2, O2, and N2 reactive gases. The inset shows the patterned Pt interdigital electrodes (Pt-IDEs) on the silicon substrate. The distance between electrode fingers was 10 μm.
4. Conclusion In this study, we have indicated that plasma surface treatment causes two changes in the SE and resistance of PAN nanofibers, without any changes in surface morphology. The increase of the polar component in the total SE promotes the formation of hydrophilic bonds, thus enhancing the surface wettability of AP-treated PAN nanofibers. From the FTIR and XPS results, it is observed that many new functional groups are produced on the surfaces of AP-treated PAN nanofibers with different reactive gases, and as a result, changes in the SE and wettability are obtained. Also, the regenerated and redeposited functional groups, which are induced by AP treatment, could affect the resistance of PAN nanofibers. After N2-plasma treatment of a PAN nanofiber, not only was an improvement in SE and wettability observed, but also a change in the electrical properties, caused by the new functional groups. Therefore, it can be expected that applications may be found in the immobilization and detection of biomolecules in electrical biosensor systems. Acknowledgment This study was financially supported by the Korea Research Foundation Grant funded by the Korean Government (KRF-2008313-D00607). Fig. 4. XPS C1s high-resolution spectra of (a) an untreated PAN nanofiber, and (b–d) PAN nanofibers treated by AP plasma with H2, O2, and N2 reactive gases, respectively (the C1s peak is deconvoluted to several peaks).
carbon double bonds [35]. We expected that the N2-plasma treatment would increase the conductivity of the PAN nanofibers by inducing the formation of many types of nitrogen bonds on the PAN surface; this is supported by the chemical composition of nitrogen in the XPS results. Although there is a difference in the resistance of about one order of magnitude between the untreated and N2-plasma-treated PAN nanofibers, this is sufficient to distinguish the change in electrical properties. Thus, these plasma-treated nanofibers could find applications in biosensors used for detecting biomaterials that become attached to them when the resistance changes. From the results of this study, we believe that the effect of plasma treatment on a PAN-based material with high conductivity will be larger than on one with low conductivity because of the rapid transfer of charge generated by the plasma through the bulk nanofiber in the former material.
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