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Gold nanoparticle decorated electrospun nanofibers: A 3D reproducible and sensitive SERS substrate
Applied Surface Science 403 (2017) 29–34
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Gold nanoparticle decorated electrospun nanofibers: A 3D reproducible and sensitive SERS substrate Zhicheng Liu a , Zhaodong Yan a , Lu Jia a , Ping Song a , Linyu Mei a , Lu Bai b,∗ , Yaqing Liu a,∗ a Shanxi Province Key Laboratory of Functional Nanocomposites, School of Materials Science and Engineering, North University of China, Taiyuan 030051, China b School of Chemical and Environmental Engineering, North University of China, Taiyuan 030051, China
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Article history: Received 19 October 2016 Received in revised form 1 January 2017 Accepted 16 January 2017 Available online 17 January 2017 Keywords: Nanoparticle Electrospinning SERS Nanocomposite Membrane
a b s t r a c t The design and fabrication of three-dimensional (3D) nanostructures with fascinating SERS performance have drawn much attention in the field of materials science and nanotechnology. In this work, 3D poly (acrylic acid) (PAA)/poly (vinyl alcohol) (PVA) nanofibrous membrane was used as templates for the electrostatic assembly of gold nanoparticles. The PAA/PVA electrospun nanofibers were crosslinked using a simple thermal treatment, which would prevent the nanofibers from dissolution in the nanoparticle solution. It was found that the gold nanoparticles were uniformly immobilized on the nanofibers, causing high reproducibility of this SERS substrate. The nanocomposite membrane also showed high sensibility for the detection of trace amount of analytes such as 4-aminothiophenol and Rhodamine 6G. © 2017 Published by Elsevier B.V.
1. Introduction Surface-enhanced Raman spectroscopy (SERS), which allows for the detection of analytes at low concentration, has emerged as one of the most promising analytical tools in the field of chemical and material science [1]. Rational design and fabrication of delicate SERS substrates has attracted considerable attention [2]. Metallic nanostructures were proved to be efficient media for SERS due to the amplification of electromagnetic fields generated by the excitation of localized surface plasmons. In general, top-down as well as bottom-up techniques were employed to generate such metallic SERS substrates [3,4]. As for the top-down techniques such as electron beam lithography and nanoimprint lithography, although precise control of the nanostructure could be realized, they are costly and time-consuming. Using the bottom-up strategy, noble metallic nanoparticles (NPs) such as gold (Au) and silver (Ag) NPs, which are widely used for SERS-active substrates, could be assembled into one-, two- and three-dimensional nanostructures [5]. Over the past two decades, there has been sustained interest in this simple and cheap approach. However, Challenges remain in constructing three-dimensional (3D) nanostructures with reproducible and sensitive SERS performance.
∗ Corresponding authors. E-mail addresses: [email protected] (L. Bai), zffl[email protected] (Y. Liu). http://dx.doi.org/10.1016/j.apsusc.2017.01.157 0169-4332/© 2017 Published by Elsevier B.V.
Electrospinning is a versatile technique employed for preparing 3D polymer nanofiber membrane [6–8]. The 3D nanofibrous membrane is formed by applying a high voltage to a capillary filled with the polymer solution to be spun, and it has the advantages of freestanding, high porosity and high surface area to volume ratio. The electrospun nanofibers with diameters ranging from the micrometer to the nanometer scale are perfect templates for the assembly of SERS-active NPs [9,10]. It is known that the NPs could be assembled either in the nanofiber or on surface of the nanofiber [9,11–14]. In the former case, the NPs and the polymer should be dissolved in the same solvent [15–18]. For example, the as-synthesized Ag NPs could be mixed with poly (vinyl alcohol) (PVA) in aqueous solution, then Ag NP dimers or aligned aggregates were found within the electrospun nanofiber which showed highly sensitive, reproducible and stable SERS properties [15]. It is worth noting that the SERS performance of the 3D electrospun SERS substrate is superior to that of the corresponding 2D cast film. Nevertheless, the probing molecules have to diffuse into the nanofiber to access the NPs, which might limit the usage of such SERS substrate. In the latter case, the NPs could be assembled onto the surface of various electrospun nanofibers by using electrostatic interaction, hydrogen bonds or covalent bonds [19–26]. Pan et al. reported that negatively-charged citrate-stabilized Ag nanocrystals could be immobilized on the electrospun nanofiber through electrostatic interaction [19]. Owing to the high density of hot spots, which
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are crucial to the SERS property, the 3D mat exhibited excellent SERS performance. Another example presented by Du et al. showed that various kinds of Au nanoparticles could be easily assembled on the surface of sulfydryl functionalized electrospun nanofiber due to the Au-S covalent bonding [23]. The density of the hot spots could be controlled by changing the amounts of the assembled NPs, and the hybrid nanocomposites also showed strong SERS enhancement. Notably, the assembly of negatively-charged NPs has been extensively investigated, probably because of the facile synthesis of these NPs such as citrated-capped NPs. The assembly of positively-charged NPs on electrospun nanofibers remains to be further explored. From the aspect of the fabrication method, multistep and delicate processes were needed for the assembly of SERS-active NPs on the eletrospun nanofibers. For example, Singamaneni et al. presented that gold nanorods could be assembled on highly aligned electrospun nanofibers [20]. In order to successfully assemble the Au nanorods, the polymer required to be both quaternized and treated with thermal crosslinking. In another example, Rabolt and coworkers showed a homogeneous and highly dense Au nanorods assembly on electrospun nanofibers [22]. Before the immobilization of Au nanorods, the nanofibers should be modified by layer-by-layer deposition of polyelectrolytes, and the minimal number of the deposition step was five. Therefore, facile and effective process is urgently required to assemble the SERS-active NPs. In this work, positively-charged Au NPs were utilized as building blocks for the preparation of 3D SERS substrates. The Au NPs were attracted by the crosslinked poly (acrylic acid) (PAA)/poly (vinyl alcohol) (PVA) electrospun nanofibers via simple electrostatic interaction, avoiding the traditional sophisticated chemical modification process which is needed for the assembly. Further experiments have revealed that this 3D fibrous nanocomposite membrane shows excellent SERS performance with high sensitivity and good reproducibility.
2. Experimental
2.3. Preparation of Au NPs decorated PAA/PVA electrospun nanofiber PAA and PVA with ratio of 1:1 were dissolved in water under magnetic stirring overnight, reaching a 15 wt% PAA/PVA homogeneous mixture solution. A certain amount of polymer solution was loaded into a syringe with a 22-gauge blunt tip needle. The flow rate was 0.3 mL h−1 , and the applied voltage was fixed at 15 kV. The collection distance was 25 cm. Under the above electrospinning conditions, PAA/PVA nanofibrous membrane was produced. In order to produce water-stable nanofibrous membranes, the PAA/PVA nanofibers were crosslinked upon heating treatment at 145 ◦ C for 30 min. Then the membranes were immersed in the Au NPs solution for 12 h to assemble the Au NPs. Eventually, the electrospun nanofibers were washed with water to remove the loosely bound Au NPs, and left to dry under ambient condition. 2.4. Characterization The Au NPs were observed by a transmission electron microscopy (TEM, JEOL JEM-1400) under an acceleration voltage of 200 kV. The optical spectrum was acquired using an UV−vis spectrophotometer (Unico UV-4802). A scanning electron microscope (SEM, JEOL JSM-6510) was used to characterize the morphologies of the membranes. The X-ray diffraction (XRD) analysis was carried out on a Haoyuan DX-2700 X-ray diffractometer. The thermogravimetric (TG) measurements were performed on a TA Q 50 thermogravimetric analyzer. The SERS properties of the PVA/PAAAu NPs hybrid nanofibers were investigated using both 4-ATP and R6G as probing molecules. 50 L of the analyte solution with varying concentrations was dropped on the as-prepared nanofiber membrane. After drying in the air for several hours, SERS measurements were conducted on the substrates. A Renishaw InVia confocal Raman spectrometer operating at 785 nm (300 mW) was used to analyze the SERS performance. The excitation power was 0.15 mW(0.05% of the maximum power), and the integral time was 10 s.
2.1. Materials
3. Results and discussion
PAA (Mw = 240000) was obtained from J&K Scientific (Beijing, China). PVA (PVA-1788) and Rhodamine 6G (R6G) were purchased from Aladdin (Shanghai, China). Other chemicals such as sodium borohydride (NaBH4 ) 4-aminothiophenol (4-ATP) and Hexadecyl trimethyl ammonium Bromide (CTAB) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). All chemicals were used without further purification.
There are two requirements to be met for templates to efficiently assemble NPs: (1) the template remains intact during the assembly process, and (2) it is easy for NPs to access the template. The wellknown 3D nanofibrous structure of the electrospun nanofibers is promising for both the assembly of NPs and the fabrication of SERS substrates [9]. Usually, the electrospun nanofibers obtained from polymers with hydrophobic nature are stable in the aqueous NP solution. However, the hydrophobicity of such nanofibers is unfavorable for the attachment of NPs in water. Thus, hydrophilic PAA/PVA nanofibers were introduced. It is obvious that the pristine PAA/PVA electrospun nanofiber could not be directly immersed into the aqueous NP solution since the fiber would dissolve in it. To overcome this shortcoming, a simple thermal treatment was utilized to crosslink the PAA/PVA nanofibers, avoiding the changing of the structure and morphology of the nanofibers. As illustrated in Fig. 1, the fabrication process of the NP assembled nanofibrous membrane consists of two main steps. The first step is the crosslinking of the pristine PAA/PVA nanofibers, and the second step is the immersion of the nanofibers into the NP solution. The whole process is simple and adaptable for the achievement of other assemblies. Although Ag NPs show higher SERS enhancement than other noble metal NPs, Au NPs instead of Ag NPs were used as building blocks here, since Au NPs are thermally, and chemically more stable than Ag NPs [28,29]. The CTAB-protected Au NPs were syn-
2.2. Synthesis of Au NPs The positively-charged CTAB-protected Au NPs were prepared using the classical seeded growth method [27]. Firstly, 2.5 × 10−4 M HAuCl4 and 2.5 × 10−4 M trisodium citrate was mixed in 20 mL aqueous solution. Then 0.6 mL of ice-cold and freshly prepared 0.1 M NaBH4 solution was added. The solution turned pink, suggesting the formation of NPs. The solution was used as seed solution for subsequent growth of Au NPs. To prepare the growth solution, 2.5 × 10−4 M HAuCl4 and 0.08 M CTAB was mixed in 100 mL aqueous solution. Subsequently, 9 mL of growth solution and 0.05 mL of 0.1 M ascorbic acid solution were mixed, and 1 mL of seed solution was added under vigorously stirring. This solution was used as seed solution for the next growth process in 30 min. The Au NPs were finally obtained after two more similar growth processes.
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Fig. 1. Schematic representation of the fabrication process of the nanofibrous nanocomposites.
Fig. 2. (a) TEM images of the CTAB-protected Au NPs. (b) UV–vis spectrum and size distribution histogram (inset) of the Au NPs.
Fig. 3. SEM images of the PAA/PVA electrospun nanofibers before (a) and after (b) the crosslinking. (c) SEM image of the Au NPs assembled PAA/PVA electrospun nanofibers. The inset is a detailed image.
thesized using the classical seeded growth method [27]. As shown in Fig. 2a, the as-prepared Au NPs had a diameter of 45.3 ± 3.9 nm. These Au NPs with uniform size were capped with cationic CTAB surfactants, which formed a bilayer on the surface of the NPs. The CTAB bilayer prevented the Au NPs from aggregation, and rendered the Au NPs positively-charged. The peak of the UV–vis spectrum (Fig. 2b) at 543 nm is due to the plasmon resonance of the Au NPs. The size distribution was relatively narrow (Inset in Fig. 2b). These positively-charged Au NPs were ready for the assembly process. The as-spun PAA/PVA nanofibers shown in Figs. 3a were continuous and uniform in diameter (237 ± 46 nm). Before the assembly of Au NPs, the pristine PAA/PVA electrospun nanofibers were crosslinked by a thermal treatment, which causes an esterification reaction between the carboxylic acid of PAA and the hydroxyl groups of PVA [30–32]. There is a slight modification of the morphology of the PAA/PVA nanofibers after the crosslinking, but the 3D nanofibrous feature remained intact (Fig. 3b). The diameter (315 ± 90 nm) of the crosslinked nanofibers is a little larger than that of the pristine ones. It is worth noting that the morphology of the crosslinked PAA/PVA nanofibers did not change any more after they were immersed into water, suggesting the crosslinked nanofibers were stable during the NP assembly. Then
the crosslinked PAA/PVA electrospun membrane was submerged into the aqueous Au NPs solution for 12 h. The color of the membrane changed from white to deep red-brown, which implies the assembly of Au NPs. As shown in Fig. 3c, the Au NPs were uniformly immobilized on the surface of the crosslinked PAA/PVA nanofibers. No obvious aggregation of the NPs was observed, which is beneficial to the reproducibility of the following SERS measurements. It should be mentioned that the 3D nanofibrous structure may facilitate the assembly process because of the enlarged surface area, which makes the NPs easy to attach on the nanofibers. In order to find out the driving force of the assembly process, the crosslinked nanofibers were dipped into a solution of negatively-charged citrate-protected Au NPs. No assembly of such NPs occurred, indicating the successful assembly of the positively-charged Au NPs was driven by electrostatic interaction. In brief, the PAA/PVA-Au NPs nanocomposite membrane with 3D nanofabrous structure was obtained. To further characterize the assembly of the Au NPs on the PAA/PVA nanofibers, XRD as well as TGA experiments were conducted. It is clear that the peaks located at 38.0◦ , 44.2◦ , 64.6◦ and 77.5◦ could be assigned to (111), (200), (220) and (311) crystallographic planes of the face-centered cubic Au crystal structure
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Fig. 4. (a) XRD spectra of the Au NPs, the crosslinked PAA/PVA nanofiber and the PAA/PVA-Au NPs nanocomposite membrane. (b) TGA curves of the crosslinked PAA/PVA nanofiber and the PAA/PVA-Au NPs nanocomposite membrane.
Fig. 5. (a) SERS spectra of 0.1 mM 4-ATP molecules collected from 20 randomly selected positions of the PAA/PVA-Au NPs nanofibers, and (b) the corresponding RSD value curve. (c) SERS mapping (1079 cm−1 ) of 4-ATP on the PAA/PVA-Au NPs nanofibers. The excitation power was 1.5 mW, and the integral time was 1 s. (d) Depth-profiling SERS spectra of 0.1 mM 4-ATP molecules collected from the PAA/PVA-Au NPs nanofibers. The interval between neighboring positions was 1 m.
(Fig. 4a), while no obvious peak was observed for the PAA/PVA nanofibers [33]. As expected, four peaks located at the same degree were found for the PAA/PVA-Au NPs nanocomposite membrane, confirming the assembly of the Au NPs. The nanofibers membranes were also examined by TG analysis in order to obtain quantitative information on the composition of the membranes. Fig. 4b shows that the main decomposition occurred from 200 to 480 ◦ C, which should be attributed to the decomposition of the PAA/PVA polymer chains and the capping CTAB molecules [30]. The estimated content of the assembled Au NPs is 16.5 wt%, which results from the strong
electrostatic interaction between the nanofibers with large surface area and the SERS-active Au NPs. Flexible SERS substrate based on electrospun nanofibers has drawn tremendous interest, since it is cost-efficient and easy to process. The SERS performance of the PAA/PVA-Au NPs nanocomposite membrane was evaluated by using 4-ATP as the probing molecule. As displayed in Fig. 5a, characteristic peaks at 1079 and 1589 cm−1 , which could be assigned to the a1 vibrational modes of 4-ATP, were observed [34–37]. It is evident that the electromagnetic field enhancement dominates the SERS performance.
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Fig. 6. (a) SERS spectra of different concentrations of 4-ATP on the PAA/PVA-Au NPs nanocomposite membrane. (b) SERS spectra of different concentrations of R6G on the PAA/PVA-Au NPs nanocomposite membrane.
Moreover, the reproducibility of the Raman signals, which is important for practical SERS measurements, was assessed by collecting SERS spectra from 20 randomly selected positions of the PAA/PVAAu NPs nanofibers (Fig. 5a). The relative standard deviation (RSD) of the SERS signals was calculated and plotted as a RSD curve according to the method reported by Lu group (Fig. 5b) [38]. Although the maximum and minimum deviations are 17.7% and 1.7%, it should be noted that the RSD values of the mainly enhanced peaks centered at 1079 and 1589 cm−1 are 5.8% and 5.6%, suggesting an excellent reproducibility of the SERS performance. Moreover, a point-bypoint SERS mapping of 4-ATP on the PAA/PVA-Au NPs nanofibers was acquired to further characterize the homogeneity of the SERS substrate (Fig. 5c). The mapping shows that the SERS enhancement is rather uniform over large area of the hybrid nanofiber membrane, which agrees with the RSD results. It is reasonable to attribute the high reproducibility of the SERS performance to the uniform distribution of the Au NPs assembled on the PAA/PVA nanofibers. As discussed above, the 3D nanofibrous structure of the hybrid membrane is important for the assembly of the Au NPs. To further clarify the effect of the 3D nanostructure on the SERS performance, depth-profiling SERS spectra were recorded as shown in Fig. 5d. There is no significant change of the SERS signals when different positions were focused, revealing that the homogeneity of the SERS enhancement exists in all three directions. Therefore, the 3D feature of the nanofibers was verified by depth-profiling SERS spectra, which could be useful in the future study. Additionally, the enhancement factor (EF) was estimated using the equation EF = (ISERS /Nads )/(Ibulk /Nbulk ) (ISERS and Ibulk are the intensity of the SERS and bulk spectra; Nads and Nbulk are the number of probing molecules adsorbed on the SERS substrate and bulk molecules excited by the laser beam.), and the EF value is about 1.2 × 105 for the PAA/PVA-Au NPs nanocomposite membrane. In order to further test the SERS sensitivity of the PAA/PVAAu NPs nanocomposite membrane, this 3D SERS substrate was employed for the detection of trace amount of analytes. As shown in Fig. 6a, The SERS intensity decreased with the decreasing concentration of 4-ATP molecules. What is more, the SERS peaks become undistinguishable when the concentration reaches 10−9 M, indicating that the lowest detection limit is 10−8 M. Besides 4-ATP molecules, commonly used R6G was chosen as probing molecules to expand the applicability of the 3D SERS substrate. The characteristic peaks located at 612, 1311, 1362 and 1509 cm−1 could be clearly detected when 10−4 M R6G solution was dropped on the membrane (Fig. 6b). The SERS signals could be clearly identified even at a concentration of 10−7 M, proving the high sensitivity of the 3D SERS substrate. It is noteworthy that the SERS sensitivity of this 3D SERS substrate is comparable to recent works, though
the fabrication process is rather simple in our case [12,39,40]. Ultimately, the high sensitivity of the hybrid membrane could arise from the numerous hot spots, which are generated from the uniformly distributed Au NPs [15]. 4. Conclusions A 3D SERS substrate based on the Au NPs decorated PAA/PVA electrospun nanofibers was achieved. No chemical modification was necessary here, while a green thermal crosslinking process was applied to fabricate water-stable nanofibers. The positivelycharged Au NPs were assembled on the PAA/PVA nanofibers through electrostatic interaction, and the NPs were uniformly distributed on the fibers. The obtained PAA/PVA-Au NPs nanocomposite membrane showed highly reproducible and sensitive SERS performance because of the 3D fibrous nanostructure and the numerous hot spots. Moreover, this nanocomposite membrane is promising in the field of sensing and detection. It is believed that other functional nanoscale positively-charged building blocks could also be assembled on the nanofibers, which might broaden the scope of application of these nanocomposite membranes. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant 21504083 and 21505123) and Shanxi Province Science Foundation for Youths (No. 2015021068 and 201601D021035). References [1] S. Schlucker, Surface-enhanced raman spectroscopy: concepts and chemical applications, Angew. Chem. Int. Ed. 53 (2014) 4756–4795. [2] M.J. Banholzer, J.E. Millstone, L.D. Qin, C.A. Mirkin, Rationally designed nanostructures for surface-enhanced Raman spectroscopy, Chem. Soc. Rev. 37 (2008) 885–897. [3] S.L. Kleinman, R.R. Frontiera, A.I. Henry, J.A. Dieringer, R.P. Van Duyne, Creating, characterizing, and controlling chemistry with sers hot spots, Phys. Chem. Chem. Phys. 15 (2013) 21–36. [4] B. Sharma, M.F. Cardinal, S.L. Kleinman, N.G. Greeneltch, R.R. Frontiera, M.G. Blaber, G.C. Schatz, R.P. Van Duyne, High-performance sers substrates: advances and challenges, MRS Bull. 38 (2013) 615–624. [5] B. Sharma, R.R. Frontiera, A.I. Henry, E. Ringe, R.P. Van Duyne, Sers: materials, applications, and the future, Mater. Today 15 (2012) 16–25. [6] W.E. Teo, S. Ramakrishna, A review on electrospinning design and nanofibre assemblies, Nanotechnology 17 (2006) R89–R106. [7] A. Greiner, J.H. Wendorff, Electrospinning: a fascinating method for the preparation of ultrathin fibres, Angew. Chem. Int. Ed. 46 (2007) 5670–5703. [8] S. Agarwal, A. Greiner, J.H. Wendorff, Functional materials by electrospinning of polymers, Prog. Polym. Sci. 38 (2013) 963–991.
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Gold nanoparticle decorated electrospun nanofibers: A 3D reproducible and sensitive SERS substrate Zhicheng Liu a , Zhaodong Yan a , Lu Jia a , Ping Song a , Linyu Mei a , Lu Bai b,∗ , Yaqing Liu a,∗ a Shanxi Province Key Laboratory of Functional Nanocomposites, School of Materials Science and Engineering, North University of China, Taiyuan 030051, China b School of Chemical and Environmental Engineering, North University of China, Taiyuan 030051, China
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Article history: Received 19 October 2016 Received in revised form 1 January 2017 Accepted 16 January 2017 Available online 17 January 2017 Keywords: Nanoparticle Electrospinning SERS Nanocomposite Membrane
a b s t r a c t The design and fabrication of three-dimensional (3D) nanostructures with fascinating SERS performance have drawn much attention in the field of materials science and nanotechnology. In this work, 3D poly (acrylic acid) (PAA)/poly (vinyl alcohol) (PVA) nanofibrous membrane was used as templates for the electrostatic assembly of gold nanoparticles. The PAA/PVA electrospun nanofibers were crosslinked using a simple thermal treatment, which would prevent the nanofibers from dissolution in the nanoparticle solution. It was found that the gold nanoparticles were uniformly immobilized on the nanofibers, causing high reproducibility of this SERS substrate. The nanocomposite membrane also showed high sensibility for the detection of trace amount of analytes such as 4-aminothiophenol and Rhodamine 6G. © 2017 Published by Elsevier B.V.
1. Introduction Surface-enhanced Raman spectroscopy (SERS), which allows for the detection of analytes at low concentration, has emerged as one of the most promising analytical tools in the field of chemical and material science [1]. Rational design and fabrication of delicate SERS substrates has attracted considerable attention [2]. Metallic nanostructures were proved to be efficient media for SERS due to the amplification of electromagnetic fields generated by the excitation of localized surface plasmons. In general, top-down as well as bottom-up techniques were employed to generate such metallic SERS substrates [3,4]. As for the top-down techniques such as electron beam lithography and nanoimprint lithography, although precise control of the nanostructure could be realized, they are costly and time-consuming. Using the bottom-up strategy, noble metallic nanoparticles (NPs) such as gold (Au) and silver (Ag) NPs, which are widely used for SERS-active substrates, could be assembled into one-, two- and three-dimensional nanostructures [5]. Over the past two decades, there has been sustained interest in this simple and cheap approach. However, Challenges remain in constructing three-dimensional (3D) nanostructures with reproducible and sensitive SERS performance.
∗ Corresponding authors. E-mail addresses: [email protected] (L. Bai), zffl[email protected] (Y. Liu). http://dx.doi.org/10.1016/j.apsusc.2017.01.157 0169-4332/© 2017 Published by Elsevier B.V.
Electrospinning is a versatile technique employed for preparing 3D polymer nanofiber membrane [6–8]. The 3D nanofibrous membrane is formed by applying a high voltage to a capillary filled with the polymer solution to be spun, and it has the advantages of freestanding, high porosity and high surface area to volume ratio. The electrospun nanofibers with diameters ranging from the micrometer to the nanometer scale are perfect templates for the assembly of SERS-active NPs [9,10]. It is known that the NPs could be assembled either in the nanofiber or on surface of the nanofiber [9,11–14]. In the former case, the NPs and the polymer should be dissolved in the same solvent [15–18]. For example, the as-synthesized Ag NPs could be mixed with poly (vinyl alcohol) (PVA) in aqueous solution, then Ag NP dimers or aligned aggregates were found within the electrospun nanofiber which showed highly sensitive, reproducible and stable SERS properties [15]. It is worth noting that the SERS performance of the 3D electrospun SERS substrate is superior to that of the corresponding 2D cast film. Nevertheless, the probing molecules have to diffuse into the nanofiber to access the NPs, which might limit the usage of such SERS substrate. In the latter case, the NPs could be assembled onto the surface of various electrospun nanofibers by using electrostatic interaction, hydrogen bonds or covalent bonds [19–26]. Pan et al. reported that negatively-charged citrate-stabilized Ag nanocrystals could be immobilized on the electrospun nanofiber through electrostatic interaction [19]. Owing to the high density of hot spots, which
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are crucial to the SERS property, the 3D mat exhibited excellent SERS performance. Another example presented by Du et al. showed that various kinds of Au nanoparticles could be easily assembled on the surface of sulfydryl functionalized electrospun nanofiber due to the Au-S covalent bonding [23]. The density of the hot spots could be controlled by changing the amounts of the assembled NPs, and the hybrid nanocomposites also showed strong SERS enhancement. Notably, the assembly of negatively-charged NPs has been extensively investigated, probably because of the facile synthesis of these NPs such as citrated-capped NPs. The assembly of positively-charged NPs on electrospun nanofibers remains to be further explored. From the aspect of the fabrication method, multistep and delicate processes were needed for the assembly of SERS-active NPs on the eletrospun nanofibers. For example, Singamaneni et al. presented that gold nanorods could be assembled on highly aligned electrospun nanofibers [20]. In order to successfully assemble the Au nanorods, the polymer required to be both quaternized and treated with thermal crosslinking. In another example, Rabolt and coworkers showed a homogeneous and highly dense Au nanorods assembly on electrospun nanofibers [22]. Before the immobilization of Au nanorods, the nanofibers should be modified by layer-by-layer deposition of polyelectrolytes, and the minimal number of the deposition step was five. Therefore, facile and effective process is urgently required to assemble the SERS-active NPs. In this work, positively-charged Au NPs were utilized as building blocks for the preparation of 3D SERS substrates. The Au NPs were attracted by the crosslinked poly (acrylic acid) (PAA)/poly (vinyl alcohol) (PVA) electrospun nanofibers via simple electrostatic interaction, avoiding the traditional sophisticated chemical modification process which is needed for the assembly. Further experiments have revealed that this 3D fibrous nanocomposite membrane shows excellent SERS performance with high sensitivity and good reproducibility.
2. Experimental
2.3. Preparation of Au NPs decorated PAA/PVA electrospun nanofiber PAA and PVA with ratio of 1:1 were dissolved in water under magnetic stirring overnight, reaching a 15 wt% PAA/PVA homogeneous mixture solution. A certain amount of polymer solution was loaded into a syringe with a 22-gauge blunt tip needle. The flow rate was 0.3 mL h−1 , and the applied voltage was fixed at 15 kV. The collection distance was 25 cm. Under the above electrospinning conditions, PAA/PVA nanofibrous membrane was produced. In order to produce water-stable nanofibrous membranes, the PAA/PVA nanofibers were crosslinked upon heating treatment at 145 ◦ C for 30 min. Then the membranes were immersed in the Au NPs solution for 12 h to assemble the Au NPs. Eventually, the electrospun nanofibers were washed with water to remove the loosely bound Au NPs, and left to dry under ambient condition. 2.4. Characterization The Au NPs were observed by a transmission electron microscopy (TEM, JEOL JEM-1400) under an acceleration voltage of 200 kV. The optical spectrum was acquired using an UV−vis spectrophotometer (Unico UV-4802). A scanning electron microscope (SEM, JEOL JSM-6510) was used to characterize the morphologies of the membranes. The X-ray diffraction (XRD) analysis was carried out on a Haoyuan DX-2700 X-ray diffractometer. The thermogravimetric (TG) measurements were performed on a TA Q 50 thermogravimetric analyzer. The SERS properties of the PVA/PAAAu NPs hybrid nanofibers were investigated using both 4-ATP and R6G as probing molecules. 50 L of the analyte solution with varying concentrations was dropped on the as-prepared nanofiber membrane. After drying in the air for several hours, SERS measurements were conducted on the substrates. A Renishaw InVia confocal Raman spectrometer operating at 785 nm (300 mW) was used to analyze the SERS performance. The excitation power was 0.15 mW(0.05% of the maximum power), and the integral time was 10 s.
2.1. Materials
3. Results and discussion
PAA (Mw = 240000) was obtained from J&K Scientific (Beijing, China). PVA (PVA-1788) and Rhodamine 6G (R6G) were purchased from Aladdin (Shanghai, China). Other chemicals such as sodium borohydride (NaBH4 ) 4-aminothiophenol (4-ATP) and Hexadecyl trimethyl ammonium Bromide (CTAB) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). All chemicals were used without further purification.
There are two requirements to be met for templates to efficiently assemble NPs: (1) the template remains intact during the assembly process, and (2) it is easy for NPs to access the template. The wellknown 3D nanofibrous structure of the electrospun nanofibers is promising for both the assembly of NPs and the fabrication of SERS substrates [9]. Usually, the electrospun nanofibers obtained from polymers with hydrophobic nature are stable in the aqueous NP solution. However, the hydrophobicity of such nanofibers is unfavorable for the attachment of NPs in water. Thus, hydrophilic PAA/PVA nanofibers were introduced. It is obvious that the pristine PAA/PVA electrospun nanofiber could not be directly immersed into the aqueous NP solution since the fiber would dissolve in it. To overcome this shortcoming, a simple thermal treatment was utilized to crosslink the PAA/PVA nanofibers, avoiding the changing of the structure and morphology of the nanofibers. As illustrated in Fig. 1, the fabrication process of the NP assembled nanofibrous membrane consists of two main steps. The first step is the crosslinking of the pristine PAA/PVA nanofibers, and the second step is the immersion of the nanofibers into the NP solution. The whole process is simple and adaptable for the achievement of other assemblies. Although Ag NPs show higher SERS enhancement than other noble metal NPs, Au NPs instead of Ag NPs were used as building blocks here, since Au NPs are thermally, and chemically more stable than Ag NPs [28,29]. The CTAB-protected Au NPs were syn-
2.2. Synthesis of Au NPs The positively-charged CTAB-protected Au NPs were prepared using the classical seeded growth method [27]. Firstly, 2.5 × 10−4 M HAuCl4 and 2.5 × 10−4 M trisodium citrate was mixed in 20 mL aqueous solution. Then 0.6 mL of ice-cold and freshly prepared 0.1 M NaBH4 solution was added. The solution turned pink, suggesting the formation of NPs. The solution was used as seed solution for subsequent growth of Au NPs. To prepare the growth solution, 2.5 × 10−4 M HAuCl4 and 0.08 M CTAB was mixed in 100 mL aqueous solution. Subsequently, 9 mL of growth solution and 0.05 mL of 0.1 M ascorbic acid solution were mixed, and 1 mL of seed solution was added under vigorously stirring. This solution was used as seed solution for the next growth process in 30 min. The Au NPs were finally obtained after two more similar growth processes.
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Fig. 1. Schematic representation of the fabrication process of the nanofibrous nanocomposites.
Fig. 2. (a) TEM images of the CTAB-protected Au NPs. (b) UV–vis spectrum and size distribution histogram (inset) of the Au NPs.
Fig. 3. SEM images of the PAA/PVA electrospun nanofibers before (a) and after (b) the crosslinking. (c) SEM image of the Au NPs assembled PAA/PVA electrospun nanofibers. The inset is a detailed image.
thesized using the classical seeded growth method [27]. As shown in Fig. 2a, the as-prepared Au NPs had a diameter of 45.3 ± 3.9 nm. These Au NPs with uniform size were capped with cationic CTAB surfactants, which formed a bilayer on the surface of the NPs. The CTAB bilayer prevented the Au NPs from aggregation, and rendered the Au NPs positively-charged. The peak of the UV–vis spectrum (Fig. 2b) at 543 nm is due to the plasmon resonance of the Au NPs. The size distribution was relatively narrow (Inset in Fig. 2b). These positively-charged Au NPs were ready for the assembly process. The as-spun PAA/PVA nanofibers shown in Figs. 3a were continuous and uniform in diameter (237 ± 46 nm). Before the assembly of Au NPs, the pristine PAA/PVA electrospun nanofibers were crosslinked by a thermal treatment, which causes an esterification reaction between the carboxylic acid of PAA and the hydroxyl groups of PVA [30–32]. There is a slight modification of the morphology of the PAA/PVA nanofibers after the crosslinking, but the 3D nanofibrous feature remained intact (Fig. 3b). The diameter (315 ± 90 nm) of the crosslinked nanofibers is a little larger than that of the pristine ones. It is worth noting that the morphology of the crosslinked PAA/PVA nanofibers did not change any more after they were immersed into water, suggesting the crosslinked nanofibers were stable during the NP assembly. Then
the crosslinked PAA/PVA electrospun membrane was submerged into the aqueous Au NPs solution for 12 h. The color of the membrane changed from white to deep red-brown, which implies the assembly of Au NPs. As shown in Fig. 3c, the Au NPs were uniformly immobilized on the surface of the crosslinked PAA/PVA nanofibers. No obvious aggregation of the NPs was observed, which is beneficial to the reproducibility of the following SERS measurements. It should be mentioned that the 3D nanofibrous structure may facilitate the assembly process because of the enlarged surface area, which makes the NPs easy to attach on the nanofibers. In order to find out the driving force of the assembly process, the crosslinked nanofibers were dipped into a solution of negatively-charged citrate-protected Au NPs. No assembly of such NPs occurred, indicating the successful assembly of the positively-charged Au NPs was driven by electrostatic interaction. In brief, the PAA/PVA-Au NPs nanocomposite membrane with 3D nanofabrous structure was obtained. To further characterize the assembly of the Au NPs on the PAA/PVA nanofibers, XRD as well as TGA experiments were conducted. It is clear that the peaks located at 38.0◦ , 44.2◦ , 64.6◦ and 77.5◦ could be assigned to (111), (200), (220) and (311) crystallographic planes of the face-centered cubic Au crystal structure
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Fig. 4. (a) XRD spectra of the Au NPs, the crosslinked PAA/PVA nanofiber and the PAA/PVA-Au NPs nanocomposite membrane. (b) TGA curves of the crosslinked PAA/PVA nanofiber and the PAA/PVA-Au NPs nanocomposite membrane.
Fig. 5. (a) SERS spectra of 0.1 mM 4-ATP molecules collected from 20 randomly selected positions of the PAA/PVA-Au NPs nanofibers, and (b) the corresponding RSD value curve. (c) SERS mapping (1079 cm−1 ) of 4-ATP on the PAA/PVA-Au NPs nanofibers. The excitation power was 1.5 mW, and the integral time was 1 s. (d) Depth-profiling SERS spectra of 0.1 mM 4-ATP molecules collected from the PAA/PVA-Au NPs nanofibers. The interval between neighboring positions was 1 m.
(Fig. 4a), while no obvious peak was observed for the PAA/PVA nanofibers [33]. As expected, four peaks located at the same degree were found for the PAA/PVA-Au NPs nanocomposite membrane, confirming the assembly of the Au NPs. The nanofibers membranes were also examined by TG analysis in order to obtain quantitative information on the composition of the membranes. Fig. 4b shows that the main decomposition occurred from 200 to 480 ◦ C, which should be attributed to the decomposition of the PAA/PVA polymer chains and the capping CTAB molecules [30]. The estimated content of the assembled Au NPs is 16.5 wt%, which results from the strong
electrostatic interaction between the nanofibers with large surface area and the SERS-active Au NPs. Flexible SERS substrate based on electrospun nanofibers has drawn tremendous interest, since it is cost-efficient and easy to process. The SERS performance of the PAA/PVA-Au NPs nanocomposite membrane was evaluated by using 4-ATP as the probing molecule. As displayed in Fig. 5a, characteristic peaks at 1079 and 1589 cm−1 , which could be assigned to the a1 vibrational modes of 4-ATP, were observed [34–37]. It is evident that the electromagnetic field enhancement dominates the SERS performance.
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Fig. 6. (a) SERS spectra of different concentrations of 4-ATP on the PAA/PVA-Au NPs nanocomposite membrane. (b) SERS spectra of different concentrations of R6G on the PAA/PVA-Au NPs nanocomposite membrane.
Moreover, the reproducibility of the Raman signals, which is important for practical SERS measurements, was assessed by collecting SERS spectra from 20 randomly selected positions of the PAA/PVAAu NPs nanofibers (Fig. 5a). The relative standard deviation (RSD) of the SERS signals was calculated and plotted as a RSD curve according to the method reported by Lu group (Fig. 5b) [38]. Although the maximum and minimum deviations are 17.7% and 1.7%, it should be noted that the RSD values of the mainly enhanced peaks centered at 1079 and 1589 cm−1 are 5.8% and 5.6%, suggesting an excellent reproducibility of the SERS performance. Moreover, a point-bypoint SERS mapping of 4-ATP on the PAA/PVA-Au NPs nanofibers was acquired to further characterize the homogeneity of the SERS substrate (Fig. 5c). The mapping shows that the SERS enhancement is rather uniform over large area of the hybrid nanofiber membrane, which agrees with the RSD results. It is reasonable to attribute the high reproducibility of the SERS performance to the uniform distribution of the Au NPs assembled on the PAA/PVA nanofibers. As discussed above, the 3D nanofibrous structure of the hybrid membrane is important for the assembly of the Au NPs. To further clarify the effect of the 3D nanostructure on the SERS performance, depth-profiling SERS spectra were recorded as shown in Fig. 5d. There is no significant change of the SERS signals when different positions were focused, revealing that the homogeneity of the SERS enhancement exists in all three directions. Therefore, the 3D feature of the nanofibers was verified by depth-profiling SERS spectra, which could be useful in the future study. Additionally, the enhancement factor (EF) was estimated using the equation EF = (ISERS /Nads )/(Ibulk /Nbulk ) (ISERS and Ibulk are the intensity of the SERS and bulk spectra; Nads and Nbulk are the number of probing molecules adsorbed on the SERS substrate and bulk molecules excited by the laser beam.), and the EF value is about 1.2 × 105 for the PAA/PVA-Au NPs nanocomposite membrane. In order to further test the SERS sensitivity of the PAA/PVAAu NPs nanocomposite membrane, this 3D SERS substrate was employed for the detection of trace amount of analytes. As shown in Fig. 6a, The SERS intensity decreased with the decreasing concentration of 4-ATP molecules. What is more, the SERS peaks become undistinguishable when the concentration reaches 10−9 M, indicating that the lowest detection limit is 10−8 M. Besides 4-ATP molecules, commonly used R6G was chosen as probing molecules to expand the applicability of the 3D SERS substrate. The characteristic peaks located at 612, 1311, 1362 and 1509 cm−1 could be clearly detected when 10−4 M R6G solution was dropped on the membrane (Fig. 6b). The SERS signals could be clearly identified even at a concentration of 10−7 M, proving the high sensitivity of the 3D SERS substrate. It is noteworthy that the SERS sensitivity of this 3D SERS substrate is comparable to recent works, though
the fabrication process is rather simple in our case [12,39,40]. Ultimately, the high sensitivity of the hybrid membrane could arise from the numerous hot spots, which are generated from the uniformly distributed Au NPs [15]. 4. Conclusions A 3D SERS substrate based on the Au NPs decorated PAA/PVA electrospun nanofibers was achieved. No chemical modification was necessary here, while a green thermal crosslinking process was applied to fabricate water-stable nanofibers. The positivelycharged Au NPs were assembled on the PAA/PVA nanofibers through electrostatic interaction, and the NPs were uniformly distributed on the fibers. The obtained PAA/PVA-Au NPs nanocomposite membrane showed highly reproducible and sensitive SERS performance because of the 3D fibrous nanostructure and the numerous hot spots. Moreover, this nanocomposite membrane is promising in the field of sensing and detection. It is believed that other functional nanoscale positively-charged building blocks could also be assembled on the nanofibers, which might broaden the scope of application of these nanocomposite membranes. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant 21504083 and 21505123) and Shanxi Province Science Foundation for Youths (No. 2015021068 and 201601D021035). References [1] S. Schlucker, Surface-enhanced raman spectroscopy: concepts and chemical applications, Angew. Chem. Int. Ed. 53 (2014) 4756–4795. [2] M.J. Banholzer, J.E. Millstone, L.D. Qin, C.A. Mirkin, Rationally designed nanostructures for surface-enhanced Raman spectroscopy, Chem. Soc. Rev. 37 (2008) 885–897. [3] S.L. Kleinman, R.R. Frontiera, A.I. Henry, J.A. Dieringer, R.P. Van Duyne, Creating, characterizing, and controlling chemistry with sers hot spots, Phys. Chem. Chem. Phys. 15 (2013) 21–36. [4] B. Sharma, M.F. Cardinal, S.L. Kleinman, N.G. Greeneltch, R.R. Frontiera, M.G. Blaber, G.C. Schatz, R.P. Van Duyne, High-performance sers substrates: advances and challenges, MRS Bull. 38 (2013) 615–624. [5] B. Sharma, R.R. Frontiera, A.I. Henry, E. Ringe, R.P. Van Duyne, Sers: materials, applications, and the future, Mater. Today 15 (2012) 16–25. [6] W.E. Teo, S. Ramakrishna, A review on electrospinning design and nanofibre assemblies, Nanotechnology 17 (2006) R89–R106. [7] A. Greiner, J.H. Wendorff, Electrospinning: a fascinating method for the preparation of ultrathin fibres, Angew. Chem. Int. Ed. 46 (2007) 5670–5703. [8] S. Agarwal, A. Greiner, J.H. Wendorff, Functional materials by electrospinning of polymers, Prog. Polym. Sci. 38 (2013) 963–991.
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