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Design of Gold nanorods Janus membrane for efficient and high-sensitive surface-enhanced Raman scattering and tunable surface plasmon resonance
Accepted Manuscript Research paper Design of Gold nanorods Janus membrane for efficient and high-sensitive surface-enhanced Raman scattering and tunable surface plasmon resonance Xiaowei Zhang, Zhenwen Zhao, Li Liu, Yunbo Li PII: DOI: Reference:
S0009-2614(19)30154-X https://doi.org/10.1016/j.cplett.2019.02.035 CPLETT 36275
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
9 November 2018 23 January 2019 18 February 2019
Please cite this article as: X. Zhang, Z. Zhao, L. Liu, Y. Li, Design of Gold nanorods Janus membrane for efficient and high-sensitive surface-enhanced Raman scattering and tunable surface plasmon resonance, Chemical Physics Letters (2019), doi: https://doi.org/10.1016/j.cplett.2019.02.035
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Li Page 1
Design of Gold nanorods Janus membrane for efficient and high-sensitive surface-enhanced Raman scattering and tunable surface plasmon resonance
Xiaowei Zhang1, Zhenwen Zhao1, Li Liu1, Yunbo Li1*
1
School of Materials Science & Engineering, Shanghai University, Shanghai 200444,
China
Corresponding author: Yunbo Li (Y. Li)*
Address: School of Materials Science & Engineering, Shanghai University, 333 Nanchen Road, Baoshan District, Shanghai 200444, China
Email: [email protected]
Li Page 2
Abstract Gold nanorods (GNRs) Janus membranes have been synthesized with thiol-terminated polyethylene glycol (PEG-SH) and thiol-terminated polystyrene (PS-SH) at the oil/water interface by modified self-assembly strategy. Compared with conventional GNRs membrane, these membranes exhibit significant surface plasmon resonance (SPR) peaks and an obvious blue shift with the addition of PEG-SH or PS-SH. Moreover, the Surface-enhanced Raman scattering (SERS) performance and reproducibility of the GNRs Janus membranes have been improved significantly. This indicates GNRs Janus membranes have great application prospects in optical applications such as science-based optical sensor design and SERS sensitive detection.
Li Page 3
1. Introduction Surface-enhanced Raman scattering (SERS) has been developed as an important method for detecting chemical and biological substances due to its high sensitivity and specificity.1 It is generally agreed that electromagnetic (EM) enhancement and chemical (CM) enhancement are the enhancement mechanisms of SERS. To EM enhancement, it can lead to localized surface plasmon resonance and large electric field enhancement near the surface of metal nanoparticles.2-4 Therefore, the SERS effect is largely due to the generation of strongly localized electromagnetic hot spots.5-8 Numerous experimental and theoretical studies have firmly shown that localized electromagnetic hot spots are closely related to the particles size and shape, interparticle spacing, and environment. 9-10 Accordingly, compared with the poor reproducibility and uneven distribution of hot spots of metal nanoparticle suspensions, metal nanofilms have a wider range of applications in SERS for their repeatability with higher SERS performance, because the nanoparticles are arranged in a two-dimensional (2D) resulting in that plasma coupling and much more enhanced EM fields can be generated at the particle junctions 11-13. The Janus membrane is an emerging interface materials that refers to 2D materials on both sides with asymmetric properties due to different components or structures,14-15 and has attracted widespread attention in the past few years. 16 The application of Janus membrane is mainly focused on ion transport. According to our previous experiments, gold nanoparticles (GNPs) Janus membrane with the hydrophilic thiol-terminal polyethylene glycol (PEG-SH) and the hydrophobic thiol-terminal polystyrene (PS-SH) could exhibit a sensitive property of SERS for determination of both hydrophilic methylene blue and hydrophobic thiram.17 Compared with GNPs, gold nanorods (GNRs), as typical anisotropic nanostructure, have stronger EM enhancement due to its two independent surface plasmon resonance (SPR) of rod effect.18-20 The
Li Page 4 transverse surface plasmon resonance (TSPR) excited perpendicular to the axial incident light is in the visible region, which is similar to the GNPs, and the other longitudinal surface plasmas resonance (LSPR) in the longer wavelength range is excited by axial polarized light. The TSPR varies slightly with the change of Janus membrane, while the position of LSPR peak in the visible and near infrared region can show a great red shift resulting in novel SERS active substrates21-23 which expands their use in optoelectronic devices24-26 and biotechnology.27-29 Nowadays, the research on GNRs membranes mostly focuses on controlling the arrangement direction of the GNRs. Although this method can improve the performance of the GNRs membrane, the experimental process is complicated. In this work, due to the strong interaction of the Au-S bond, Janus structure of the GNRs membrane was designed by introducing hydrophilic PEG-SH and hydrophobic PS-SH, and developed a modified self-assembly strategy at the oil/water interface for the fabrication of GNRs Janus membrane. Compared with conventional GNRs membrane, TSPR and LSPR absorption peaks were investigated on the Janus membranes by adjusting addition of PEG-SH and PS-SH. Moreover, the addition of PS-SH and PEG-SH change the spacing between GNRs was also studied on dense SERS hot spots and SERS activity of GNRs Janus membranes was systematically compared with GNPs Janus membranes.
2. Experimental section Materials Hexadecyl trimethyl ammonium bromide (CTAB, >99.0%), sodium oleate (NaOL), tetrachloroauric acid hydrate (HAuCl4 •4H2O, 99.99%), silver nitrate (AgNO3, >99.0%), sodium borohydride (NaBH4, 99.0%), hydrochloric acid (HCl, 37.0 wt % in water), ascorbic acid (AA, 99.7%), acetone, isoamyl acetate, hexane and ethanol were obtained from Sinopharm Chemical Reagent
Co.,
Ltd.
Thiol-terminated
polyethylene
glycol
(PEG-SH,
Mn=6000)
and
Li Page 5 thiol-terminated polystyrene (PS-SH, Mn=11000) were purchased from Sigma-Aldrich. Methylene blue was purchased from Aladdin. All solutions and dilutions were prepared using deionized water. Synthesis of Gold nanorods The gold nanorods (GNRs) were synthesized based on the previously reported method. 30 The growth solution was prepared as follows: 1.75 g of CTAB and 0.31 g of NaOL were dissolved in 62.5 mL of deionized water and heated to 50 oC. 6.0 ml of 4.0 mM AgNO3 solution was added when the solution was cooled to 30 oC. The mixture was kept undisturbed at 30 oC for 15 min after 62.5 mL of 1.0 mM HAuCl4 solution was added. After 60 min，the solution became colorless and 0.75 ml of HCl was introduced. After another 15 min of slow stirring, 0.31 ml of 0.064 M ascorbic acid (AA) was added and the solution was vigorously stirred for 0.5 min. To prepare the seed solution, 5.0 mL of 0.2 M CTAB solution was mixed with 5.0 mL of 0.5 mM HAuCl4 in a 20 mL scintillation vial. Then, 1.0 ml of 0.1 M NaBH4 was injected into the mixed solution under vigorous stirring and the stirring lasted 2.0 min. The seed solution was aged at 30 o
C for 30 min before use. Eventually, 0.1 mL seed solution was injected into the growth solution.
The resultant mixture was stirred for 0.5 min and left undisturbed at 30 oC for 17 h for GNRs growth. The final products were isolated by centrifugation at 2500 rpm for 5.0 min and the sediment was discarded to remove the impurities. The remained supernatant was centrifuged at 8000 rpm for 45 min followed by removal of the supernatant. The procedure was repeated three times to remove as much of the CTAB as possible and the precipitate (about 200 μL) was diluted to 0.5 mL with deionized water. Fabrication of GNRs and Janus membranes
Li Page 6 The detailed procedures for fabricating GNRs membrane, PEG-GNRs membrane, PS-GNRs Janus membrane and PS-GNRs-PEG Janus membrane were shown in Fig. 1 respectively. These membranes were prepared in a rectangular glass cell (4.0 cm×3.0 cm×3.0 cm) by self-assembly strategy at the oil/water interface. 31-32 The oil phase (4.0 ml) was a mixture of hexane and isoamyl acetate (Vheptane/Visoamyl acetate = 3:1). Different kinds of GNRs solution (4.0 ml) were the aqueous phase. 2.5 ml of ethanol was slowly injected into the oil/water interface which led to the GNRs arrange into membrane at the oil/water interface by reducing the electrostatic repulsion between the GNRs.33 The oil phase gradually volatilized and the membrane could be picked up with slide glass or silicon wafer when the oil phase was completely volatilized.
Fig. 1 Schematic representation of (a) GNRs membrane, (b) PEG-GNRs membrane, (c) PS-GNRs Janus membrane, (d) PS-GNRs-PEG Jamus membrane self-assembly at oil/water interface.
The prepared and unmodified GNRs solution (4.0 ml) was used as the aqueous phase, together with the mixed oil phase (4.0 ml) that has been prepared before. The GNRs membrane was obtained by injecting ethanol at the oil/water interface (Fig. 1a). The hydrophilic PEG-SH was
Li Page 7 dissolved in deionized water to get different concentrations of PEG-SH aqueous solution. Already-prepared GNRs solution (3.0 ml) was mixed with the PEG-SH aqueous solution (0.7 mg/ml, 1.4 mg/ml, 2.1 mg/ml and 3.5 mg/ml, 1 ml). The mixture was gently stirred at the room temperature for 24 h to get PEGylated GNRs aqueous solution which was the aqueous phase. PEG-GNRs membrane was obtained as previously described (Fig. 1b). Hydrophobic PS-SH was dissolved in the mixed oil (4.0 ml) to obtain different concentrations (0.05 mg/ml, 0.25 mg/ml, 0.5 mg/ml) of oil phase, and the prepared GNRs solution (4.0 ml) was used as the aqueous phase. The GNRs arrange into a membrane at the oil/water interface by injecting ethanol. At the same time, with the volatilization of the oil and the action of Au-S, PS-SH was adsorbed on the GNRs membrane and the PS-GNRs Janus membrane was obtained (Fig. 1c). In addition, the mixed oil phase with PS-SH (0.25 mg/ml, 4.0 ml) and the aqueous phasee with PEGylated GNRs (0.7 mg/ml, 4.0 ml) were injected to the rectangular glass cell to achieve PS-GNRs-PEG Janus membranes which the top face (facing to the oil phase) is attached with hydrophobic PS-SH (Fig. 1d).34 Characterizations Ultraviolet-visible-near-infrared (UV-VIS-NIR) absorption spectra were recorded with a U-4150 spectrophotometer. Scanning electron microscopy (SEM) images of the nanofilms were obtained using a JEOL JSM-6700F microscope at 5.0 kV. The SERS property of the samples on silicon wafers were studied using Renishaw INVIA Raman spectrometer with a laser at excitation wavelength of 633 nm, laser power was 2 mW, and the exposure time was 10 s. The light spot size was approximately 1.5 μm2 and optical focus was adjusted manually to probe different positions of substrates.
Li Page 8
3. Results and discussion
Absorbance (a.u.)
a
975nm
1432 nm
GNRs membrane 792 nm 506 nm
GNRs colloid
400
800
1200
1600
2000
Wavelength (nm)
Fig. 2 (a) Absorption spectra of GNRs colloids and GNRs membrane on slide glass, (b) SEM image of GNRs membrane. Fig. 2a shows the UV-VIS-NIR absorption spectra of GNRs colloid and GNRs membrane. The spectrum of GNRs colloid shows two distinct absorption bands (TSPR and LSPR of GNRs) centered at 506 nm and 975 nm, respectively. The absorption bands of GNRs colloid were quite narrow, indicating the monodispersity GNRs. Compared with GNRs colloid, the positions of the two absorption peaks of GNRs membrane show an obvious red-shift, moving to 792 nm and 1432 nm, respectively. And the half width of the peaks becomes wider, representing the absorption range is broaden. According to relevant theoretical and experimental reports, this is attributed to the anisotropy of the GNRs and the strong surface plasma dipole coupling between the tightly adjacent GNRs. The SEM image (Fig. 2b) of GNRs membrane shows the uniform GNRs without significant aggregation, containing a close-packed nanoparticle. The diameter and the length of GNRs are around 11.5 nm and 63.4 nm.
Li Page 9
a
1440
Wavelength (nm)
Absorbance (a.u.)
5 4 3 2 1
1400
b
1360
LSPR
1320 840 780
c
720
TSPR
660 600 500
1000
1500
2000
Wavelength (nm)
2500
0.0
0.7
1.4
2.1
2.8
3.5
The proportion of PEG-SH
Fig. 3 (a) Absorption spectra of PEG-GNRs membranes coated on slide glass with the varying proportion of PEG-SH (1: 0 mg/ml, 2: 0.7 mg/ml, 3: 1.4 mg/ml, 4: 2.1 mg/ml, 5: 3.5 mg/ml), (b) the peak position of LSPR of the PEG-GNRs membranes as a function of the proportion of PEG-SH, (c) the peak position of TSPR of the PEG-GNRs membranes as a function of the proportion of PEG-SH.
Fig. 3a shows the absorption spectra of PEG-GNRs membranes coated on slide glass with the varying proportion of PEG-SH (1: 0 mg/ml, 2: 0.7 mg/ml, 3: 1.4 mg/ml, 4: 2.1 mg/ml, 5: 3.5 mg/ml). Due to the larger curvature of the ends of GNRs, CTAB is more adsorbed on the side of the GNRs.35 In addition, the Au-S bond is much stronger than the electrostatic interaction between GNRs and CTAB.36-37 Therefore, most of the PEG-SH is preferentially adsorbed at both ends of the GNRs to form PEG-GNRs.23 With the addition of PEG-SH, both the resonance absorption peaks of the PEG-GNRs membrane undergo continuous blue shift and the TSPR absorption peak is more obvious. The blue-shift is due to the interaction of PEG changes the spatial array of GNRs. That is, PEG chains between GNRs expand the spacing between adjacent GNRs.
Li Page 10 Fig. 3b and Fig. 3c show in detail the wavelength changes of the LSPR absorption peak (1440 nm to 1300 nm) and the TSPR absorption peak (820 nm to 580 nm) with the addition of PEG-SH. The TSPR absorption peak has a blue shift greater than that of the LSPR absorption peak, because the transverse surface of the GNRs absorbs more PEG-SH.38 Moreover, the addition of PEG-SH narrows the half-width peak of the membrane, indicating that the arrangement of GNRs is more regular.
a
1440 Wavelength (nm)
Absorbance (a.u.)
4 3 2 1
1360
b LSPR
1280 1200 840
c
770 700
TSPR
630 500
1000
1500
2000
Wavelength (nm)
2500
0.0
0.1
0.2
0.3
0.4
0.5
The proportion of PS-SH
Fig. 4 (a) Absorption spectra of PS-GNRs Janus membranes coated on slide glass with the varying proportion of PS-SH (1: 0 mg/ml, 2: 0.05 mg/ml, 3: 0.25 mg/ml, 4: 0.5 mg/ml), (b) the peak position of LSPR of the PS-GNRs Janus membranes as a function of the proportion of PS-SH, (c) the peak position of TSPR of the PS-GNRs Janus membranes as a function of the proportion of PS-SH.
The resonance absorption spectrum of the PS-GNRs Janus membrane is shown in Fig. 4a. It was found that the TSPR absorption peak and the LSPR absorption peak of GNRs were blue-shifted with the addition of PS-SH. PS-GNRs Janus membrane is formed by biphase self-assembly method. The hydrophobic PS-SH in the organic solvent can replace the CTAB on the surface of the GNRs in the aqueous phase due to the strong interaction of Au-S.
Li Page 11 Fig. 4b and 4c show the relationship between the intensity of LSPR absorption peak (and the TSPR absorption peak) of the PS-GNRs Janus membranes and the PS-SH content in detail. When a small amount of PS-SH (0.05 mg/ml) was added, the TSPR absorption peak of the membrane showed a weak blue shift from 812 nm to 806 nm, but the LSPR absorption peak did not change. However, with the increase of PS-SH content (0.25 mg/ml, 0.5 mg/ml), the TSPR absorption peaks (673 nm, 625 nm) and LSPR absorption peaks (1278 nm, 1167 nm) of the membrane had a clear continuous blue shift. It is contribute to the interaction between the PS-SH and the GNRs array increases as the PS-SH content increases.39 The spacing between the GNRs in the membrane becomes larger.
a Absorbance (a.u.)
b 1 2
3 4
500
1000
1500
2000
2500
Wavelength (nm)
Fig. 5 (a) Absorption spectra of GNRs membrane (1), PEG-GNRs membrane (2: 0.7 mg/ml PEG-SH), PS-GNRs Janus membrane (3: 0.25 mg/m PS-SH) and PS-GNRs-PEG Janus membrane (4: 0.25 mg/ml PS-SH and 0.7 mg/ml PEG-SH) coated on slide glass, (b) Optical photo images of GNRs membrane, PEG-GNRs membrane, PS-GNRs Janus membrane and PS-GNRs-PEG Janus membrane coated on slide glass.
Li Page 12 Fig. 5a shows the absorption spectra of four different structures of GNRs membranes, which indicates that these two SPR absorption peaks of the membrane can be further blue-shifted when both PS-SH and PEG-SH are added. The absorption spectra of Fig. 3 and Fig. 4 indicate that the SPR peak of the GNRs membranes can be gradually blue-shifted with the PEG-SH and PS-SH addition. Therefore, the SPR peaks of PS-GNRs-PEG Janus membrane (PS: 0.25 mg/ml, PEG-SH: 0.7 mg/ml) can be blue-shifted too. Photo images of the four different structures of GNRs membranes (Fig. 5b) also show the significant color change due to the blue-shift of the
PEG-GNRs membrane GNRs membrane 522 cm-1 Blank
800
1200
Raman Shift (cm-1)
1600
PS-GNRs-PEG Janus membrane
PS-GNRs Janus membrane
PS-GNRs Janus membrane
446 cm-1 PS-GNRs-PEG Janus membrane
400
b
1396 cm-1 1625 cm-1
PEG-GNRs membrane
5104
Peak Integration
Raman intensity (a.u.)
a
GNRs membrane
SPR peaks.
2000
GNRs membranes with different structure
Fig. 6 (a) Comparison of SERS spectra of MB adsorbed on silicon wafer, GNRs membrane, PEG-GNR membrane, PS-GNRs Janus membrane and PS-GNRs-PEG membrane, (b) Raman band intensity and error bar graph of gold membranes with different structures at 1625cm -1 of MB.
As described in Figure 1, the membranes were transferred onto the silicon wafer to prepare the SERS substrates. The membranes were soaked in aqueous solutions of 10−6 M methylene blue (MB) solution for 4 hours for adsorption of the MB molecules, then taken out and dried in air.
Li Page 13 The SERS of MB (10-6 M) on the different GNRs membranes were measured at the excitation of 633 nm. The SERS signals of bare silicon wafer (blank), GNRs membrane, PEG-GNRs membrane, PS-GNRs Janus membrane and PS-GNRs-PEG Janus membrane were listed as shown in Fig. 6a. Three Raman enhancement peaks of MB can be clearly seen in the SERS spectrum because the surrounding electromagnetic field is enhanced due to the strong coupling between GNRs in the GNRs membrane. The peak at 447 cm-1 is due to C-N-C vibration. The bands at 1395 cm-1 and 1625 cm-1 are attributed to the asymmetric contraction vibration of the C-C bond and the in-plane deformation of the C-H bond. The Raman peaks of MB are almost invisible on the pure silicon substrate without the GNRs mambrane and only the Raman peak of silicon at 522 cm-1 can be obtained. To further evaluate the reproducibility of the membrane as a SERS substrate, a plurality of positions were selected for each SERS substrate for measurement, and errors were calculated based on the average and standard deviation. 40 Eight different sites were tested for each of the membranes here and error analysis was performed. Fig. 6b was calculated on the change in Raman intensity at 1625 cm-1. In combination with Fig. 6a and Fig. 6b, the enhanced signal intensities of PEG-GNRs membrane and GNRs membrane are similar. The enhancement signal of PS-GNRs Janus membrane is slightly higher than that of PEG-GNRs membrane and GNRs membrane. The error of each point of GNRs membrane is larger due to the anisotropy of GNRs whereas the errors of PS-GNRs Janus membrane and PEG-GNRs membrane are significantly smaller. The sharp signal enhancement of the PS-GNRs-PEG Janus membrane is significantly higher in 2-4 times than that of the other three GNRs membrane. The addition of PS-SH and PEG-SH can adjust the spacing between GNRs and make the arrangement of GNRs more regular
Li Page 14 so that its error of PS-GNRs-PEG Janus membrane is minimal, indicating that the SERS hot spots distribution of the membrane was more uniform. In order to quantify the Raman enhancement effect of these substrates, the following formula was used to calculate the Raman enhancement factor (EF) of the substrate surface. 41 EF
I SERS / N SERS I bulk / Nbulk
（1）
where ISERS is the peak integration of the enhanced Raman scattering at a certain chemical band, and Ibulk is the integration of the normal Raman spectra collected from MB on silicon wafer at the corresponding chemical band. The in-plane deformation of the C-H bond at 1625 cm−1 was chosen as the model band to calculate the EF. NSERS and Nbulk represent the number of MB excited by a laser beam in SERS and normal Raman scattering, respectively. NSERS and Nbulk use the following simplified algorithm 42 N SERS nSERS N A
VSERS CSERS NA S SERS
（2）
Vbulk Cbulk NA Sbulk
（3）
Nbulk nbulk N A
Where nSERS represents a certain volume (VSERS) and concentration (CSERS) of MB ethanol solution was dispersed onto an area of SSERS on the GNRs membranes. nbulk represents that a certain volume (Vbulk) and concentration (Cbulk) of MB ethanol solution was dispersed onto an area of Sbulk at a clean silicon wafer substrate. NA represents Avogadro constant. In our experiment, 50 μL of 10-2 M MB ethanol solution was dispersed onto an area of about 25 mm2 on silicon wafer to record the Raman spectrum, and 20 μL of 10-6 M MB ethanol solution was dispersed onto an area of about 25 mm2 on GNRs membranes for SERS spectra.
Li Page 15 Table 1
SERS EF of GNRs membranes and GNPs membranes for MB
Membranes
GNRs EF (MB)
GNPs EF (MB)17
Gold nanomembrane
4.39×107
7.69×103
PS-Gold Janus nanomembrane
1.37×108
3.13×104
PS-Gold-PEG Janus nanomembrane
6.41×108
2.86×106
The EF of different kinds of GNRs membranes are significantly higher than those of the same kind of GNPs membranes (Table 1) due to the excellent surface plasmon resonance (SPR) properties of GNRs. Particularly, the EF of PS-GNRs-PEG Janus membrane is 224 times than that of PS-GNPs-PEG Janus membrane. In addition, the EF of PEG-GNRs membrane (4.87×107) is almost the same as that of GNRs membrane (4.39×107), which is consistent with Fig. 6a. EF of the PS-GNRs-PEG Janus membrane and PS-GNRs Janus membrane for MB are 6.41×108 and 1.37×108, which are higher than that of GNRs membrane.
4. Conclusions The GNRs Janus membranes were synthesized by modified self-assembly strategy at the oil/water interface with thiol-terminated polymers. The SPR peaks of the GNRs membrane exhibited a significant red-shift, compared with the corresponding GNRs colloid. However, the membranes showed two distinct SPR peaks and displayed a continuous blue shift phenomenon with addition of PEG-SH or PS-SH. The SERS performance and reproducibility of PS-GNRs-PEG Janus membrane were obviously higher than those of other GNRs membranes and the SERS EF of the it reaches 6.41 × 108. It demonstrates that the hybrid structure of GNRs membrane can be changed by using thiol-terminated polymers which adjusts the characteristics of SPR of GNRs membrane and greatly improves SERS activity. This indicates GNRs Janus
Li Page 16 membranes have great application prospects in optical applications such as science-based optical sensor design and SERS sensitive detection.
Conflicts of interest There are no conflicts to declare.
Acknowledgements The authors acknowledge the National Natural Science Foundation of China (Grant No. 51203088).
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Li Page 19
Highlight 1. Gold nanorods Janus membranes have been synthesized with thiol-terminated polyethylene glycol (PEG-SH) and thiol-terminated polystyrene (PS-SH) at the oil/water interface by modified self-assembly strategy. 2. Compared with conventional gold nanorod membrane, these membranes exhibit significant surface plasmas resonance peaks and an obvious blue shift with addition of PEG-SH or PS-SH. 3. The Surface-enhanced Raman scattering (SERS) performance and stability of the gold nanorods Janus membranes have been improved significantly. 4. This indicates GNRs Janus membranes have great application prospects in optical applications such as science-based optical sensor design and SERS sensitive detection.
Li Page 20
Graphical Abstract Comparison of sensitive surface-enhanced Raman scattering spectra of MB adsorbed on Gold
Raman intensity (a.u.)
nanorods (GNRs) Janus membranes
a
5104
1396 cm-1 1625 cm-1
446 cm-1 PS-GNRs-PEG Janus film PS-GNRs Janus film GNRs-PEG film GNRs film Blank 400
800
1200
1600 -1
Raman Shift (cm )
2000
S0009-2614(19)30154-X https://doi.org/10.1016/j.cplett.2019.02.035 CPLETT 36275
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
9 November 2018 23 January 2019 18 February 2019
Please cite this article as: X. Zhang, Z. Zhao, L. Liu, Y. Li, Design of Gold nanorods Janus membrane for efficient and high-sensitive surface-enhanced Raman scattering and tunable surface plasmon resonance, Chemical Physics Letters (2019), doi: https://doi.org/10.1016/j.cplett.2019.02.035
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Li Page 1
Design of Gold nanorods Janus membrane for efficient and high-sensitive surface-enhanced Raman scattering and tunable surface plasmon resonance
Xiaowei Zhang1, Zhenwen Zhao1, Li Liu1, Yunbo Li1*
1
School of Materials Science & Engineering, Shanghai University, Shanghai 200444,
China
Corresponding author: Yunbo Li (Y. Li)*
Address: School of Materials Science & Engineering, Shanghai University, 333 Nanchen Road, Baoshan District, Shanghai 200444, China
Email: [email protected]
Li Page 2
Abstract Gold nanorods (GNRs) Janus membranes have been synthesized with thiol-terminated polyethylene glycol (PEG-SH) and thiol-terminated polystyrene (PS-SH) at the oil/water interface by modified self-assembly strategy. Compared with conventional GNRs membrane, these membranes exhibit significant surface plasmon resonance (SPR) peaks and an obvious blue shift with the addition of PEG-SH or PS-SH. Moreover, the Surface-enhanced Raman scattering (SERS) performance and reproducibility of the GNRs Janus membranes have been improved significantly. This indicates GNRs Janus membranes have great application prospects in optical applications such as science-based optical sensor design and SERS sensitive detection.
Li Page 3
1. Introduction Surface-enhanced Raman scattering (SERS) has been developed as an important method for detecting chemical and biological substances due to its high sensitivity and specificity.1 It is generally agreed that electromagnetic (EM) enhancement and chemical (CM) enhancement are the enhancement mechanisms of SERS. To EM enhancement, it can lead to localized surface plasmon resonance and large electric field enhancement near the surface of metal nanoparticles.2-4 Therefore, the SERS effect is largely due to the generation of strongly localized electromagnetic hot spots.5-8 Numerous experimental and theoretical studies have firmly shown that localized electromagnetic hot spots are closely related to the particles size and shape, interparticle spacing, and environment. 9-10 Accordingly, compared with the poor reproducibility and uneven distribution of hot spots of metal nanoparticle suspensions, metal nanofilms have a wider range of applications in SERS for their repeatability with higher SERS performance, because the nanoparticles are arranged in a two-dimensional (2D) resulting in that plasma coupling and much more enhanced EM fields can be generated at the particle junctions 11-13. The Janus membrane is an emerging interface materials that refers to 2D materials on both sides with asymmetric properties due to different components or structures,14-15 and has attracted widespread attention in the past few years. 16 The application of Janus membrane is mainly focused on ion transport. According to our previous experiments, gold nanoparticles (GNPs) Janus membrane with the hydrophilic thiol-terminal polyethylene glycol (PEG-SH) and the hydrophobic thiol-terminal polystyrene (PS-SH) could exhibit a sensitive property of SERS for determination of both hydrophilic methylene blue and hydrophobic thiram.17 Compared with GNPs, gold nanorods (GNRs), as typical anisotropic nanostructure, have stronger EM enhancement due to its two independent surface plasmon resonance (SPR) of rod effect.18-20 The
Li Page 4 transverse surface plasmon resonance (TSPR) excited perpendicular to the axial incident light is in the visible region, which is similar to the GNPs, and the other longitudinal surface plasmas resonance (LSPR) in the longer wavelength range is excited by axial polarized light. The TSPR varies slightly with the change of Janus membrane, while the position of LSPR peak in the visible and near infrared region can show a great red shift resulting in novel SERS active substrates21-23 which expands their use in optoelectronic devices24-26 and biotechnology.27-29 Nowadays, the research on GNRs membranes mostly focuses on controlling the arrangement direction of the GNRs. Although this method can improve the performance of the GNRs membrane, the experimental process is complicated. In this work, due to the strong interaction of the Au-S bond, Janus structure of the GNRs membrane was designed by introducing hydrophilic PEG-SH and hydrophobic PS-SH, and developed a modified self-assembly strategy at the oil/water interface for the fabrication of GNRs Janus membrane. Compared with conventional GNRs membrane, TSPR and LSPR absorption peaks were investigated on the Janus membranes by adjusting addition of PEG-SH and PS-SH. Moreover, the addition of PS-SH and PEG-SH change the spacing between GNRs was also studied on dense SERS hot spots and SERS activity of GNRs Janus membranes was systematically compared with GNPs Janus membranes.
2. Experimental section Materials Hexadecyl trimethyl ammonium bromide (CTAB, >99.0%), sodium oleate (NaOL), tetrachloroauric acid hydrate (HAuCl4 •4H2O, 99.99%), silver nitrate (AgNO3, >99.0%), sodium borohydride (NaBH4, 99.0%), hydrochloric acid (HCl, 37.0 wt % in water), ascorbic acid (AA, 99.7%), acetone, isoamyl acetate, hexane and ethanol were obtained from Sinopharm Chemical Reagent
Co.,
Ltd.
Thiol-terminated
polyethylene
glycol
(PEG-SH,
Mn=6000)
and
Li Page 5 thiol-terminated polystyrene (PS-SH, Mn=11000) were purchased from Sigma-Aldrich. Methylene blue was purchased from Aladdin. All solutions and dilutions were prepared using deionized water. Synthesis of Gold nanorods The gold nanorods (GNRs) were synthesized based on the previously reported method. 30 The growth solution was prepared as follows: 1.75 g of CTAB and 0.31 g of NaOL were dissolved in 62.5 mL of deionized water and heated to 50 oC. 6.0 ml of 4.0 mM AgNO3 solution was added when the solution was cooled to 30 oC. The mixture was kept undisturbed at 30 oC for 15 min after 62.5 mL of 1.0 mM HAuCl4 solution was added. After 60 min，the solution became colorless and 0.75 ml of HCl was introduced. After another 15 min of slow stirring, 0.31 ml of 0.064 M ascorbic acid (AA) was added and the solution was vigorously stirred for 0.5 min. To prepare the seed solution, 5.0 mL of 0.2 M CTAB solution was mixed with 5.0 mL of 0.5 mM HAuCl4 in a 20 mL scintillation vial. Then, 1.0 ml of 0.1 M NaBH4 was injected into the mixed solution under vigorous stirring and the stirring lasted 2.0 min. The seed solution was aged at 30 o
C for 30 min before use. Eventually, 0.1 mL seed solution was injected into the growth solution.
The resultant mixture was stirred for 0.5 min and left undisturbed at 30 oC for 17 h for GNRs growth. The final products were isolated by centrifugation at 2500 rpm for 5.0 min and the sediment was discarded to remove the impurities. The remained supernatant was centrifuged at 8000 rpm for 45 min followed by removal of the supernatant. The procedure was repeated three times to remove as much of the CTAB as possible and the precipitate (about 200 μL) was diluted to 0.5 mL with deionized water. Fabrication of GNRs and Janus membranes
Li Page 6 The detailed procedures for fabricating GNRs membrane, PEG-GNRs membrane, PS-GNRs Janus membrane and PS-GNRs-PEG Janus membrane were shown in Fig. 1 respectively. These membranes were prepared in a rectangular glass cell (4.0 cm×3.0 cm×3.0 cm) by self-assembly strategy at the oil/water interface. 31-32 The oil phase (4.0 ml) was a mixture of hexane and isoamyl acetate (Vheptane/Visoamyl acetate = 3:1). Different kinds of GNRs solution (4.0 ml) were the aqueous phase. 2.5 ml of ethanol was slowly injected into the oil/water interface which led to the GNRs arrange into membrane at the oil/water interface by reducing the electrostatic repulsion between the GNRs.33 The oil phase gradually volatilized and the membrane could be picked up with slide glass or silicon wafer when the oil phase was completely volatilized.
Fig. 1 Schematic representation of (a) GNRs membrane, (b) PEG-GNRs membrane, (c) PS-GNRs Janus membrane, (d) PS-GNRs-PEG Jamus membrane self-assembly at oil/water interface.
The prepared and unmodified GNRs solution (4.0 ml) was used as the aqueous phase, together with the mixed oil phase (4.0 ml) that has been prepared before. The GNRs membrane was obtained by injecting ethanol at the oil/water interface (Fig. 1a). The hydrophilic PEG-SH was
Li Page 7 dissolved in deionized water to get different concentrations of PEG-SH aqueous solution. Already-prepared GNRs solution (3.0 ml) was mixed with the PEG-SH aqueous solution (0.7 mg/ml, 1.4 mg/ml, 2.1 mg/ml and 3.5 mg/ml, 1 ml). The mixture was gently stirred at the room temperature for 24 h to get PEGylated GNRs aqueous solution which was the aqueous phase. PEG-GNRs membrane was obtained as previously described (Fig. 1b). Hydrophobic PS-SH was dissolved in the mixed oil (4.0 ml) to obtain different concentrations (0.05 mg/ml, 0.25 mg/ml, 0.5 mg/ml) of oil phase, and the prepared GNRs solution (4.0 ml) was used as the aqueous phase. The GNRs arrange into a membrane at the oil/water interface by injecting ethanol. At the same time, with the volatilization of the oil and the action of Au-S, PS-SH was adsorbed on the GNRs membrane and the PS-GNRs Janus membrane was obtained (Fig. 1c). In addition, the mixed oil phase with PS-SH (0.25 mg/ml, 4.0 ml) and the aqueous phasee with PEGylated GNRs (0.7 mg/ml, 4.0 ml) were injected to the rectangular glass cell to achieve PS-GNRs-PEG Janus membranes which the top face (facing to the oil phase) is attached with hydrophobic PS-SH (Fig. 1d).34 Characterizations Ultraviolet-visible-near-infrared (UV-VIS-NIR) absorption spectra were recorded with a U-4150 spectrophotometer. Scanning electron microscopy (SEM) images of the nanofilms were obtained using a JEOL JSM-6700F microscope at 5.0 kV. The SERS property of the samples on silicon wafers were studied using Renishaw INVIA Raman spectrometer with a laser at excitation wavelength of 633 nm, laser power was 2 mW, and the exposure time was 10 s. The light spot size was approximately 1.5 μm2 and optical focus was adjusted manually to probe different positions of substrates.
Li Page 8
3. Results and discussion
Absorbance (a.u.)
a
975nm
1432 nm
GNRs membrane 792 nm 506 nm
GNRs colloid
400
800
1200
1600
2000
Wavelength (nm)
Fig. 2 (a) Absorption spectra of GNRs colloids and GNRs membrane on slide glass, (b) SEM image of GNRs membrane. Fig. 2a shows the UV-VIS-NIR absorption spectra of GNRs colloid and GNRs membrane. The spectrum of GNRs colloid shows two distinct absorption bands (TSPR and LSPR of GNRs) centered at 506 nm and 975 nm, respectively. The absorption bands of GNRs colloid were quite narrow, indicating the monodispersity GNRs. Compared with GNRs colloid, the positions of the two absorption peaks of GNRs membrane show an obvious red-shift, moving to 792 nm and 1432 nm, respectively. And the half width of the peaks becomes wider, representing the absorption range is broaden. According to relevant theoretical and experimental reports, this is attributed to the anisotropy of the GNRs and the strong surface plasma dipole coupling between the tightly adjacent GNRs. The SEM image (Fig. 2b) of GNRs membrane shows the uniform GNRs without significant aggregation, containing a close-packed nanoparticle. The diameter and the length of GNRs are around 11.5 nm and 63.4 nm.
Li Page 9
a
1440
Wavelength (nm)
Absorbance (a.u.)
5 4 3 2 1
1400
b
1360
LSPR
1320 840 780
c
720
TSPR
660 600 500
1000
1500
2000
Wavelength (nm)
2500
0.0
0.7
1.4
2.1
2.8
3.5
The proportion of PEG-SH
Fig. 3 (a) Absorption spectra of PEG-GNRs membranes coated on slide glass with the varying proportion of PEG-SH (1: 0 mg/ml, 2: 0.7 mg/ml, 3: 1.4 mg/ml, 4: 2.1 mg/ml, 5: 3.5 mg/ml), (b) the peak position of LSPR of the PEG-GNRs membranes as a function of the proportion of PEG-SH, (c) the peak position of TSPR of the PEG-GNRs membranes as a function of the proportion of PEG-SH.
Fig. 3a shows the absorption spectra of PEG-GNRs membranes coated on slide glass with the varying proportion of PEG-SH (1: 0 mg/ml, 2: 0.7 mg/ml, 3: 1.4 mg/ml, 4: 2.1 mg/ml, 5: 3.5 mg/ml). Due to the larger curvature of the ends of GNRs, CTAB is more adsorbed on the side of the GNRs.35 In addition, the Au-S bond is much stronger than the electrostatic interaction between GNRs and CTAB.36-37 Therefore, most of the PEG-SH is preferentially adsorbed at both ends of the GNRs to form PEG-GNRs.23 With the addition of PEG-SH, both the resonance absorption peaks of the PEG-GNRs membrane undergo continuous blue shift and the TSPR absorption peak is more obvious. The blue-shift is due to the interaction of PEG changes the spatial array of GNRs. That is, PEG chains between GNRs expand the spacing between adjacent GNRs.
Li Page 10 Fig. 3b and Fig. 3c show in detail the wavelength changes of the LSPR absorption peak (1440 nm to 1300 nm) and the TSPR absorption peak (820 nm to 580 nm) with the addition of PEG-SH. The TSPR absorption peak has a blue shift greater than that of the LSPR absorption peak, because the transverse surface of the GNRs absorbs more PEG-SH.38 Moreover, the addition of PEG-SH narrows the half-width peak of the membrane, indicating that the arrangement of GNRs is more regular.
a
1440 Wavelength (nm)
Absorbance (a.u.)
4 3 2 1
1360
b LSPR
1280 1200 840
c
770 700
TSPR
630 500
1000
1500
2000
Wavelength (nm)
2500
0.0
0.1
0.2
0.3
0.4
0.5
The proportion of PS-SH
Fig. 4 (a) Absorption spectra of PS-GNRs Janus membranes coated on slide glass with the varying proportion of PS-SH (1: 0 mg/ml, 2: 0.05 mg/ml, 3: 0.25 mg/ml, 4: 0.5 mg/ml), (b) the peak position of LSPR of the PS-GNRs Janus membranes as a function of the proportion of PS-SH, (c) the peak position of TSPR of the PS-GNRs Janus membranes as a function of the proportion of PS-SH.
The resonance absorption spectrum of the PS-GNRs Janus membrane is shown in Fig. 4a. It was found that the TSPR absorption peak and the LSPR absorption peak of GNRs were blue-shifted with the addition of PS-SH. PS-GNRs Janus membrane is formed by biphase self-assembly method. The hydrophobic PS-SH in the organic solvent can replace the CTAB on the surface of the GNRs in the aqueous phase due to the strong interaction of Au-S.
Li Page 11 Fig. 4b and 4c show the relationship between the intensity of LSPR absorption peak (and the TSPR absorption peak) of the PS-GNRs Janus membranes and the PS-SH content in detail. When a small amount of PS-SH (0.05 mg/ml) was added, the TSPR absorption peak of the membrane showed a weak blue shift from 812 nm to 806 nm, but the LSPR absorption peak did not change. However, with the increase of PS-SH content (0.25 mg/ml, 0.5 mg/ml), the TSPR absorption peaks (673 nm, 625 nm) and LSPR absorption peaks (1278 nm, 1167 nm) of the membrane had a clear continuous blue shift. It is contribute to the interaction between the PS-SH and the GNRs array increases as the PS-SH content increases.39 The spacing between the GNRs in the membrane becomes larger.
a Absorbance (a.u.)
b 1 2
3 4
500
1000
1500
2000
2500
Wavelength (nm)
Fig. 5 (a) Absorption spectra of GNRs membrane (1), PEG-GNRs membrane (2: 0.7 mg/ml PEG-SH), PS-GNRs Janus membrane (3: 0.25 mg/m PS-SH) and PS-GNRs-PEG Janus membrane (4: 0.25 mg/ml PS-SH and 0.7 mg/ml PEG-SH) coated on slide glass, (b) Optical photo images of GNRs membrane, PEG-GNRs membrane, PS-GNRs Janus membrane and PS-GNRs-PEG Janus membrane coated on slide glass.
Li Page 12 Fig. 5a shows the absorption spectra of four different structures of GNRs membranes, which indicates that these two SPR absorption peaks of the membrane can be further blue-shifted when both PS-SH and PEG-SH are added. The absorption spectra of Fig. 3 and Fig. 4 indicate that the SPR peak of the GNRs membranes can be gradually blue-shifted with the PEG-SH and PS-SH addition. Therefore, the SPR peaks of PS-GNRs-PEG Janus membrane (PS: 0.25 mg/ml, PEG-SH: 0.7 mg/ml) can be blue-shifted too. Photo images of the four different structures of GNRs membranes (Fig. 5b) also show the significant color change due to the blue-shift of the
PEG-GNRs membrane GNRs membrane 522 cm-1 Blank
800
1200
Raman Shift (cm-1)
1600
PS-GNRs-PEG Janus membrane
PS-GNRs Janus membrane
PS-GNRs Janus membrane
446 cm-1 PS-GNRs-PEG Janus membrane
400
b
1396 cm-1 1625 cm-1
PEG-GNRs membrane
5104
Peak Integration
Raman intensity (a.u.)
a
GNRs membrane
SPR peaks.
2000
GNRs membranes with different structure
Fig. 6 (a) Comparison of SERS spectra of MB adsorbed on silicon wafer, GNRs membrane, PEG-GNR membrane, PS-GNRs Janus membrane and PS-GNRs-PEG membrane, (b) Raman band intensity and error bar graph of gold membranes with different structures at 1625cm -1 of MB.
As described in Figure 1, the membranes were transferred onto the silicon wafer to prepare the SERS substrates. The membranes were soaked in aqueous solutions of 10−6 M methylene blue (MB) solution for 4 hours for adsorption of the MB molecules, then taken out and dried in air.
Li Page 13 The SERS of MB (10-6 M) on the different GNRs membranes were measured at the excitation of 633 nm. The SERS signals of bare silicon wafer (blank), GNRs membrane, PEG-GNRs membrane, PS-GNRs Janus membrane and PS-GNRs-PEG Janus membrane were listed as shown in Fig. 6a. Three Raman enhancement peaks of MB can be clearly seen in the SERS spectrum because the surrounding electromagnetic field is enhanced due to the strong coupling between GNRs in the GNRs membrane. The peak at 447 cm-1 is due to C-N-C vibration. The bands at 1395 cm-1 and 1625 cm-1 are attributed to the asymmetric contraction vibration of the C-C bond and the in-plane deformation of the C-H bond. The Raman peaks of MB are almost invisible on the pure silicon substrate without the GNRs mambrane and only the Raman peak of silicon at 522 cm-1 can be obtained. To further evaluate the reproducibility of the membrane as a SERS substrate, a plurality of positions were selected for each SERS substrate for measurement, and errors were calculated based on the average and standard deviation. 40 Eight different sites were tested for each of the membranes here and error analysis was performed. Fig. 6b was calculated on the change in Raman intensity at 1625 cm-1. In combination with Fig. 6a and Fig. 6b, the enhanced signal intensities of PEG-GNRs membrane and GNRs membrane are similar. The enhancement signal of PS-GNRs Janus membrane is slightly higher than that of PEG-GNRs membrane and GNRs membrane. The error of each point of GNRs membrane is larger due to the anisotropy of GNRs whereas the errors of PS-GNRs Janus membrane and PEG-GNRs membrane are significantly smaller. The sharp signal enhancement of the PS-GNRs-PEG Janus membrane is significantly higher in 2-4 times than that of the other three GNRs membrane. The addition of PS-SH and PEG-SH can adjust the spacing between GNRs and make the arrangement of GNRs more regular
Li Page 14 so that its error of PS-GNRs-PEG Janus membrane is minimal, indicating that the SERS hot spots distribution of the membrane was more uniform. In order to quantify the Raman enhancement effect of these substrates, the following formula was used to calculate the Raman enhancement factor (EF) of the substrate surface. 41 EF
I SERS / N SERS I bulk / Nbulk
（1）
where ISERS is the peak integration of the enhanced Raman scattering at a certain chemical band, and Ibulk is the integration of the normal Raman spectra collected from MB on silicon wafer at the corresponding chemical band. The in-plane deformation of the C-H bond at 1625 cm−1 was chosen as the model band to calculate the EF. NSERS and Nbulk represent the number of MB excited by a laser beam in SERS and normal Raman scattering, respectively. NSERS and Nbulk use the following simplified algorithm 42 N SERS nSERS N A
VSERS CSERS NA S SERS
（2）
Vbulk Cbulk NA Sbulk
（3）
Nbulk nbulk N A
Where nSERS represents a certain volume (VSERS) and concentration (CSERS) of MB ethanol solution was dispersed onto an area of SSERS on the GNRs membranes. nbulk represents that a certain volume (Vbulk) and concentration (Cbulk) of MB ethanol solution was dispersed onto an area of Sbulk at a clean silicon wafer substrate. NA represents Avogadro constant. In our experiment, 50 μL of 10-2 M MB ethanol solution was dispersed onto an area of about 25 mm2 on silicon wafer to record the Raman spectrum, and 20 μL of 10-6 M MB ethanol solution was dispersed onto an area of about 25 mm2 on GNRs membranes for SERS spectra.
Li Page 15 Table 1
SERS EF of GNRs membranes and GNPs membranes for MB
Membranes
GNRs EF (MB)
GNPs EF (MB)17
Gold nanomembrane
4.39×107
7.69×103
PS-Gold Janus nanomembrane
1.37×108
3.13×104
PS-Gold-PEG Janus nanomembrane
6.41×108
2.86×106
The EF of different kinds of GNRs membranes are significantly higher than those of the same kind of GNPs membranes (Table 1) due to the excellent surface plasmon resonance (SPR) properties of GNRs. Particularly, the EF of PS-GNRs-PEG Janus membrane is 224 times than that of PS-GNPs-PEG Janus membrane. In addition, the EF of PEG-GNRs membrane (4.87×107) is almost the same as that of GNRs membrane (4.39×107), which is consistent with Fig. 6a. EF of the PS-GNRs-PEG Janus membrane and PS-GNRs Janus membrane for MB are 6.41×108 and 1.37×108, which are higher than that of GNRs membrane.
4. Conclusions The GNRs Janus membranes were synthesized by modified self-assembly strategy at the oil/water interface with thiol-terminated polymers. The SPR peaks of the GNRs membrane exhibited a significant red-shift, compared with the corresponding GNRs colloid. However, the membranes showed two distinct SPR peaks and displayed a continuous blue shift phenomenon with addition of PEG-SH or PS-SH. The SERS performance and reproducibility of PS-GNRs-PEG Janus membrane were obviously higher than those of other GNRs membranes and the SERS EF of the it reaches 6.41 × 108. It demonstrates that the hybrid structure of GNRs membrane can be changed by using thiol-terminated polymers which adjusts the characteristics of SPR of GNRs membrane and greatly improves SERS activity. This indicates GNRs Janus
Li Page 16 membranes have great application prospects in optical applications such as science-based optical sensor design and SERS sensitive detection.
Conflicts of interest There are no conflicts to declare.
Acknowledgements The authors acknowledge the National Natural Science Foundation of China (Grant No. 51203088).
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Highlight 1. Gold nanorods Janus membranes have been synthesized with thiol-terminated polyethylene glycol (PEG-SH) and thiol-terminated polystyrene (PS-SH) at the oil/water interface by modified self-assembly strategy. 2. Compared with conventional gold nanorod membrane, these membranes exhibit significant surface plasmas resonance peaks and an obvious blue shift with addition of PEG-SH or PS-SH. 3. The Surface-enhanced Raman scattering (SERS) performance and stability of the gold nanorods Janus membranes have been improved significantly. 4. This indicates GNRs Janus membranes have great application prospects in optical applications such as science-based optical sensor design and SERS sensitive detection.
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Graphical Abstract Comparison of sensitive surface-enhanced Raman scattering spectra of MB adsorbed on Gold
Raman intensity (a.u.)
nanorods (GNRs) Janus membranes
a
5104
1396 cm-1 1625 cm-1
446 cm-1 PS-GNRs-PEG Janus film PS-GNRs Janus film GNRs-PEG film GNRs film Blank 400
800
1200
1600 -1
Raman Shift (cm )
2000