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CNT nanohybrids for the SERS application
Applied Surface Science 487 (2019) 1077–1083
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Suspended 3D AgNPs/CNT nanohybrids for the SERS application a
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Chundong Liu , Lei Wang , Yu Guo , Xu Gao , Yuanyuan Xu , Qin Wei , Baoyuan Man , ⁎⁎ Cheng Yanga,d, a
School of Physics and Electronics, Shandong Normal University, Jinan 250014, PR China Shandong Sanjian Construction Projects Stock Co., Ltd., Jinan 250010, PR China School of Science, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, PR China d Institute of Materials and Clean Energy, Shandong Normal University, Jinan 250014, PR China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: AgNPs/CNTs nanohybrids Suspended SERS
There dimensional (3D) metallic nanohybrids have attracted attentions for the applications in nanophotonic devices, catalysis and biosensor. Here, the 3D suspended Ag nanoparticles/Carbon nanotubes (AgNPs/CNTs) nanohybrids, obtained by self-aggregating AgNPs onto the suspended CNT networks, are synthesized and used for the SERS application. The electric field distribution with the near-field and far-field coupling can be obtained by the optimization of the nanogaps of the AgNPs. Such suspended AgNPs/CNTs nanohybrids are studied to be used as the ultra-sensitive SERS biosensor, benefiting from the bio-compatibility and chemical absorption of the CNT networks and the EM enhancement of the 3D denser “hotspots”. R6G molecules with a concentration as low as 10−15 M can be clearly detected, demonstrating their excellent Raman enhancement.
1. Introduction Currently, metal nanostructures are widely concerned due to the localized surface plasmon resonance (LSPR) generated by the collective oscillations of electron gas in it [1,2]. When LSPR is excited, both absorption and scattering of the light beam interacted with metal nanostructures are greatly enhanced. In addition, the extremely intense electromagnetic fields caused by the LSPR provide a sensitive probe to detect the small changes in dielectric environment around nanostructures, which is especially attractive for sensing applications [3–5]. Based on these traits, 3D metallic nanostructures with densely distribution and suitable nanogaps have attracted attentions for the applications of photonic devices [6–8], catalysis [9,10] and biosensor [11–15]. The optical properties of such metal nanostructures are highly desired for various applications, including molecular spectroscopy [16–18], surface plasmon resonance (SPR), sensing, therapy, imaging and photocatalysis [19]. Metallic nanohybrids, combined by the metal nanostructures and low-dimensional materials, are effective to form the dense distribution and suitable nanogaps. Low-dimensional materials have also attracted wide spread attentions due to their large specific surface area, high compressive strength and good electrical conductivity [20,21]. Simultaneously, its large specific surface area and high compressive
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strength make it easy to load nanoparticles [22]. Zhang synthesized the sheet-like NiO/CNT nanohybrids via a solvothermal method [23]; WanHo Chung et al. presented a way to make hybrid nanostructured transparent conductive films based on the assembly of metal nanowires and graphene films [24]. Here, we provided a convenient and efficient method for constructing suspended 3D AgNPs/CNT nanohybrids. Suspended CNTs networks were firstly obtained and then loaded by AgNPs with a diameter of ~75 nm to form such 3D AgNPs/CNT nanohybrids, which have both near-field and far-field effects [25]. In addition, CNTs with good biocompatibility and adsorbability have attracted much attention as an excellent fluorescence quenching platform to make optical, biological and chemical sensors [26]. Surfaceenhanced Raman scattering (SERS) as an analytical technique is widely used due to its advantages of high sensitivity and non-invasion in the field of biology and chemistry [27–29]. The electromagnetic mechanism (EM) and chemical mechanism (CM) have been widely recognized as two main enhancement mechanisms of SERS [30]. EM can make the enhancement up to 1014 caused by the surface plasmon resonance [31,32] and CM provides additional enhancement by a charge transfer process [33–35]. Therefore, combination of the both two mechanisms is meaningful. In the work of Sun, the nanoporous surface for SERS substrate was fabricated in batches and at low costs by cross stacking superaligned carbon nanotube films [36]; Zhang et al. chose
Corresponding author. Correspondence to: C. Yang, Institute of Materials and Clean Energy, Shandong Normal University, Jinan 250014, PR China. E-mail addresses: [email protected] (B. Man), [email protected] (C. Yang).
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https://doi.org/10.1016/j.apsusc.2019.05.179 Received 15 March 2019; Received in revised form 27 April 2019; Accepted 15 May 2019 Available online 17 May 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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AgNPs of different size and morphology via magnetron sputtering or high-temperature annealing, and assembled them on the surface and sidewalls of the aligned carbon nanotube (CNT) arrays to get a highly sensitive stable SERS substrate [37,38]. In our experiment, the Raman enhancement factor (EF) as high as 4.88 × 1011 was obtained, which demonstrates that this substrate has a satisfactory enhancement effect. The numerical simulations also prove the superior performance of these nanohybrids.
2.4. Numerical analysis of the electric field distribution
2. Experimental section
2.5. The SERS experiments
2.1. Materials
Rhodamine 6G (R6G) molecules are used as the target molecules for the SERS experiments. R6G powder was dissolved in deionized water to form the different concentration solution. R6G sample was dried up before the SERS test. 15 randomly selected spots on the substrate are measured and averaged to obtain the Raman spectra at each concentration. Raman spectroscopy was collected using a Horiba HR evolution 800 Raman microscope system with 532 nm laser (0.48 mW) with a spot size of 1um.
The electric field properties of samples were numerically calculated by the commercial finite difference time domain (FDTD) method. The size, morphology and structure of the samples were extracted by reference to the scanning electron microscope (SEM) images. The laser with a wavelength of 532 nm was used as the incident laser here, which was perpendicularly incident the sample.
Acetone (CH3COCH3, 99.5%), alcohol (C2H6O, 99.7%), Rhodamine 6G (R6G), and silver nitrate (AgNO3) were purchased from local chemical plant. Polyvinylpyrrolidone (PVP, Mw = 55,000) was purchased from Sigma-Aldrich. All chemicals were used without further purification. 2.2. Assembly of AgNPs
3. Results and discussions
The AgNPs used here were obtained by reducing AgNO3, which was mainly based on the method introduced by Zhang et al. [39]. Typically, 0.25 g PVP and 0.05 g AgNO3 were dissolved into the 20 mL ethylene glycol. The mixture was heated at 135 °C for 2 h.
Suspended AgNPs/CNTs nanohybrids were designed (Fig. 2(a)) and formed (Fig. 2(b)) in our experiment, the electric field distribution (Fig. 2(c)–(d)) with the near and far-field coupling were simulated by using the FDTD simulation. It can be clearly seen that the AgNPs density on the CNT nets is significantly higher than that on the silicon substrate, which proves that the AgNPs are more tend to adhere to the CNTs. The CNTs were firm enough as the carriers to support metal nanoparticles. AgNPs adhered on the surface of CNTs densely and uniformly to generate strong localized surface plasmon resonance (LSPR). Besides, the specific nano-cavities formed by the 3D network structure enabled the incident laser to oscillate, which was beneficial for electrical and optical application. The periodic-distribution array as well as the organized distribution of CNTs and AgNPs all enable the substrates to be a uniform test platform. To analyze and explain the performance of the structure, the electric field distributions and absorption features are numerically simulated via the FDTD method. The diameter of CNTs and the size of AgNPs used in this simulation are respectively set as 2 nm and 80 nm, dense CNTs can be considered as a thin layer. For this structure, the density of the AgNPs is conveniently regulated. Moreover, as the structural density changes, its enhancement mechanism also changes. Therefore, we designed two models with different density for analysis. For the compact AgNPs/CNT/AgNPs structure, the gaps
2.3. Preparation of the suspended AgNPs/CNT nanohybrids The Preparation of the nanohybrids is shown in Fig. 1. Suspended CNTs were firstly synthesized according to Liu's work [40]. A 100 nm Pd array was lithographically patterned on a SiO2/Si wafer, and then a 400 nm Ag film was deposited on the array. After this procedure, a layer of CNTs was deposited on the Ag film surface by the chemical vapor deposition (CVD). The ferrocene/sulfur powder was flowed into the growth zone along with the mixed gas of 1000 sccm argon and 10 sccm methane at a temperature of 1100 °C. At a specified time, the CNTs would be deposited on the surface of substrate placed in the deposition zone at a temperature of 150 °C. During the deposition, the density of CNTs was proportional to deposition time. Followed by annealing at 960 °C, the Ag films became to melt and aggregate towards the Pd array. The CNTs were followed with the Ag migration to the two ends of the Pd columns. Finally, 3 μL AgNPs solution was dropped onto the suspended CNT substrate by a pipette and dried at 80 °C in a nitrogen atmosphere.
Fig. 1. The synthesized processes of the suspended AgNPs/CNT nanohybrids. 1078
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Fig. 2. (a) Sketch map of the three-dimensional nanohybrids model. (b) SEM image of the nanohybrids. (c, d) The simulated electric field distributions of dense AgNPs (near-field) and sparsely arranged AgNPs (far-field).
measured on the substrate after the addition of AgNPs is compared with the previous one (Fig. 3(h)). The Raman peak of the CNT can still be clearly seen in the figure, and the heights of these peaks are significantly higher than previous one, which proves that the CNTs still exist after the process of dropping AgNPs solution, and also confirmed the Raman enhancement induced by AgNPs. It is noteworthy that the ratio of the height of the D peak at 1350 cm−1 to the height of the G peak at 1580 cm−1 (ID/IG) is significantly increased, which generally means an increase in defects of the CNT surface. It may be caused by a large amount of AgNPs adhering to the CNT surface. For exploring optimal AgNPs concentration for the SERS application, 3 μL AgNPs solution was dropped with the different times. Fabricated nanohybrids were shown in Fig. 4(a)–(e) respectively. It can be clearly seen that the density of AgNPs increased significantly with the increasing of the number of deposition times. Moreover, the AgNPs could be effectively intercepted and assembled along the CNTs to form suspending three-dimensional “hot spots”. Besides the excellent toughness property of CNTs, both ends of these CNTs were also embedded in the outer Ag layer of Pd. These properties of CNTs ensured that a certain amount of AgNPs can be caught by the CNTs networks. At the same time, it also proved that the substrate was sturdy enough to avoid the fracture of CNTs during the Raman experiments. R6G molecules, which are the common probe molecules, were used here to analyze the performance of such SERS substrates. R6G solution with a concentration of 10−7 M was selected to explore the Raman enhancement of the suspended 3D AgNPs/CNTs substrates. Fig. 5(a) and (b) show the Raman spectra of R6G on five substrates with different AgNPs concentration, which shows that an trend from rise to decline of the SERS signal are obtained with the increasing of the dropping times. With the concentration of the AgNPs increasing, the Raman intensity of R6G increases significantly, which can be attributed to the increasing of the AgNPs “hot spots”. It should be mentioned that the incident laser can be oscillated on the nanogaps and the layers of the AgNPs. The additional coupling of the laser on the layers of the AgNPs, which is caused by the cavity of the suspended CNTs network, can greatly enhance the effects of LSPR, as shown in the Fig. 5(c). However, as the concentration of AgNPs is further increased, AgNPs becomes to be aggregated, the number of the “hot spots” is decreased, cavity is much smaller, and the 3D nanostructures becomes to be vanished, so that the
between adjacent AgNPs are ca. 2 nm, and for the sparse AgNPs/CNT/ AgNPs structure, the intervals are set ca. 30 nm. Moreover, in the latter situation, there is 150 nm of distance from the bi-level CNTs. The numerical data for refractive index of C and Ag are respectively obtained from the previous literature [41,42]. In this simulation, the perfectly matched layer boundary condition and periodic boundary condition are respectively used in z and x(y) directions, and the mesh sizes are set as 0.2 nm in all the directions. As shown in Fig. 2(c), the strong electric field is mainly distributed in the nanogap between AgNPs and AgNPs. In its top layer, the maximum E/E0 is as high as 40. For the case with the larger nanogaps (Fig. 2(d)), the AgNPs between different layers form plasma coupling due to the far-field effect. It can be seen from the results of our simulation that even in the sixth layer of AgNPs, the E/E0 is still as high as 2.8. Therefore, the suspended AgNPs/CNTs nanohybrids can be expected to suit various fields because it has both strong LSPR effect and plasma coupling effect. CNTs can be successfully suspended between Pd pillars, which could be clearly observed in Fig. 3(a) and (b). Raman spectra were collected from random ten regions across the entire substrate, which was displayed in Fig. 3(c). It can be seen clearly that the peaks intensity of CNTs at various points was quite close to each other, which demonstrated the excellent uniformity of the substrate. The slight difference on intensity of Raman peaks at different location was mainly due to the little difference of the CNT number. We chose the G peak of CNT at 1580 cm−1 to measure the Raman mapping, and it shows the relatively smooth and uniform color distribution with only a little dark region (Fig. 3(d)), which indicates the prepared CNTs array is highly homogenous. AgNPs with the diameter of ~75 nm (Fig. 3(e)) were chosen here to be loaded onto the suspended CNTs networks to achieve that: 1) the suspended CNTs networks will not be broken; 2) the electric field distribution can be easily controlled on the near-field or far-field coupling (shown in Fig. 2(c)–(d)). For the formation of the suspended AgNPs/ CNTs, 3 μL solution of chemically synthesized AgNPs (1.05 g/mL) was dropped directly onto the substrate. The substrates were then heated at 80 °C to consolidate the AgNPs onto the CNTs. The AgNPs solution should be added dropwise to form the suspended nanohybrids. So that, AgNPs were self-aggregated onto the surface of suspended CNTs successfully, which is shown in Fig. 3(f)–(g). The Raman spectrum 1079
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Fig. 3. (a), (b) The SEM images of the suspended CNTs. (c) Raman spectra of the CNTs array. (d) Raman mapping of the peak at 1580 cm−1 over 20 μm × 20 μm area. (e) SEM image of the AgNPs on the silicon sheet. (f), (g) SEM image of the different concentrations of AgNPs self-aggregated on the CNT surface. (h) Comparison of Raman spectra of AgNPs/CNT and CNT.
specific surface area of the substrate and forms more three-dimensional hot spots, so its Raman enhancement effect is still higher than that of the AgNPs on the silicon substrate. At the same time, when more AgNPs cover the Pd array (Fig. 4(f)), the intensity of the measured Raman peak is quite close to that on the silicon substrate, which also proves our analysis. To demonstrate the SERS sensitivity, SERS spectra of R6G molecules with various concentrations are collected in Fig. 6(a). When the concentration is as low as 10−15 M, the 613, 774, 1185, 1315, 1365, 1508 and 1650 cm−1 peaks can still be easily observed (inset of the Fig. 6(b)). The 613 cm−1 peaks were selected to express the linear relation of R6G. As shown in Fig. 6(b), the coefficient of determination is as high as 0.987. This result proves that the substrate shows good Raman enhancement and excellent linearity. Because the AgNPs/CNTs nanohybrids was mainly dealing with average SERS signals in the Raman experiment, the detailed
Raman intensity of R6G is decreased significantly. To prove the advantage of this hybrid structure of the suspended 3D AgNPs/CNTs substrates, the various enhancement mechanisms should be analyzed one by one. Highest enhancement obtained on the 3D AgNPs/CNT structure due to the electromagnetic enhancement of the AgNPs and the chemical enhancement of the CNTs and the adsorption of the molecules by the suspended structures. The Raman spectra of the AgNPs on the polished silicon planes are detected in the same condition (shown in Fig. 5(d)). Comparing the enhancement by measuring the intensity of the characteristic peaks at 613 cm−1 in the Raman spectrum of R6G, it can be seen that the enhancement effect of the suspended 3D AgNPs/ CNTs substrate is about three times of the AgNPs on polished silicon planes. When the AgNPs are added in excess, the suspended 3D nanostructures are covered (Fig. 4(e)), and the enhancement of the substrate is mainly provided by the electromagnetic enhancement of the flat AgNPs. However, the presence of the Pd column array increases the
Fig. 4. SEM images of the suspended CNT (a) and the suspended AgNPs/CNTs with the different dropping times ((b) once, (c) twice, (d) three times, (e) four times and (f) five times). 1080
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Fig. 5. (a) Raman spectra of R6G on the five substrates with the different dropping times. (b) Raman intensity of the R6G peaks at 613, 773 and 1508 cm−1 on the five different substrates. (c) The additional coupling of the laser on the layers of the AgNPs. (d) The Raman spectra of the R6G on the suspended 3D AgNPs/CNTs and the silicon substrates (inset: SEM image of the AgNPs on the polished silicon planes).
NSERS roughly equals to the ratio of R6G concentration. Therefore, we adopted the approximate value of CRS/CSERS as the value of NRS/NSERS to obtain estimated EF. In the experiment, the intensity of the characteristic peaks at 613 cm−1 in the Raman spectrum of R6G with the concentration of 10−15 M is about 80. It has been calculated that the intensity of R6G peaks at 613 cm−1 with 10−3 M on the pure SiO2 substrate is 149.6. Hence, the ISERS/IRS and the NSERS/NRS can be calculated 0.53 and 1012. The EF on the substrate can be estimated as 5.35 × 1011, which proves that our substrate has excellent Raman enhancement. Homogeneity and reproducibility are important for the practical
distribution of the EF on the substrate, or even its maximum value, is irrelevant. The SERS enhancement factors (EF) for R6G on the suspended 3D AgNPs/CNTs substrates was calculated according to the standard equation:
EF =
ISERS / NSERS IRS / NRS
Here, ISERS and IRS, according to the previous reports [43], represent the peaks intensities of SERS spectra and the normal Raman spectra respectively. NSERS and NRS respectively represent the corresponding numbers of molecules within the incident laser spot. The value of NRS/
Fig. 6. (a) Raman spectra of the R6G with various concentrations. (b) Linear relation of R6G peaks intensities as a function of concentration at 613 cm−1 (inset: the Raman spectrum of R6G with the concentration of 10−15 M). 1081
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Fig. 7. (a) The Raman spectra measured randomly at 15 different locations. (b) Intensity of the 613 cm−1 peaks as a function of position.
SERS application. The large-area uniformly distributed suspended 3D AgNPs/CNTs nanohybrids are considered to have excellent uniformity. The SERS spectra of R6G molecules at 10−9 M from 15 random spots were recorded in Fig. 7(a), the results showed that the peak value of corresponding position is greatly consistent, the intensities for each peak also fluctuate quite mildly. The 613 cm−1 peaks were selected to intuitive investigate the uniformity (Fig. 7(b)). The 613 cm−1 peaks intensity at all positions display a small fluctuation near the average value (black solid line). The fluctuation ranges are restricted in the region from −6.3% to +9.8%, which demonstrates an excellent uniformity of the Raman signals.
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Chundong Liu , Lei Wang , Yu Guo , Xu Gao , Yuanyuan Xu , Qin Wei , Baoyuan Man , ⁎⁎ Cheng Yanga,d, a
School of Physics and Electronics, Shandong Normal University, Jinan 250014, PR China Shandong Sanjian Construction Projects Stock Co., Ltd., Jinan 250010, PR China School of Science, Qilu University of Technology (Shandong Academy of Sciences), Jinan 250353, PR China d Institute of Materials and Clean Energy, Shandong Normal University, Jinan 250014, PR China b c
A R T I C LE I N FO
A B S T R A C T
Keywords: AgNPs/CNTs nanohybrids Suspended SERS
There dimensional (3D) metallic nanohybrids have attracted attentions for the applications in nanophotonic devices, catalysis and biosensor. Here, the 3D suspended Ag nanoparticles/Carbon nanotubes (AgNPs/CNTs) nanohybrids, obtained by self-aggregating AgNPs onto the suspended CNT networks, are synthesized and used for the SERS application. The electric field distribution with the near-field and far-field coupling can be obtained by the optimization of the nanogaps of the AgNPs. Such suspended AgNPs/CNTs nanohybrids are studied to be used as the ultra-sensitive SERS biosensor, benefiting from the bio-compatibility and chemical absorption of the CNT networks and the EM enhancement of the 3D denser “hotspots”. R6G molecules with a concentration as low as 10−15 M can be clearly detected, demonstrating their excellent Raman enhancement.
1. Introduction Currently, metal nanostructures are widely concerned due to the localized surface plasmon resonance (LSPR) generated by the collective oscillations of electron gas in it [1,2]. When LSPR is excited, both absorption and scattering of the light beam interacted with metal nanostructures are greatly enhanced. In addition, the extremely intense electromagnetic fields caused by the LSPR provide a sensitive probe to detect the small changes in dielectric environment around nanostructures, which is especially attractive for sensing applications [3–5]. Based on these traits, 3D metallic nanostructures with densely distribution and suitable nanogaps have attracted attentions for the applications of photonic devices [6–8], catalysis [9,10] and biosensor [11–15]. The optical properties of such metal nanostructures are highly desired for various applications, including molecular spectroscopy [16–18], surface plasmon resonance (SPR), sensing, therapy, imaging and photocatalysis [19]. Metallic nanohybrids, combined by the metal nanostructures and low-dimensional materials, are effective to form the dense distribution and suitable nanogaps. Low-dimensional materials have also attracted wide spread attentions due to their large specific surface area, high compressive strength and good electrical conductivity [20,21]. Simultaneously, its large specific surface area and high compressive
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strength make it easy to load nanoparticles [22]. Zhang synthesized the sheet-like NiO/CNT nanohybrids via a solvothermal method [23]; WanHo Chung et al. presented a way to make hybrid nanostructured transparent conductive films based on the assembly of metal nanowires and graphene films [24]. Here, we provided a convenient and efficient method for constructing suspended 3D AgNPs/CNT nanohybrids. Suspended CNTs networks were firstly obtained and then loaded by AgNPs with a diameter of ~75 nm to form such 3D AgNPs/CNT nanohybrids, which have both near-field and far-field effects [25]. In addition, CNTs with good biocompatibility and adsorbability have attracted much attention as an excellent fluorescence quenching platform to make optical, biological and chemical sensors [26]. Surfaceenhanced Raman scattering (SERS) as an analytical technique is widely used due to its advantages of high sensitivity and non-invasion in the field of biology and chemistry [27–29]. The electromagnetic mechanism (EM) and chemical mechanism (CM) have been widely recognized as two main enhancement mechanisms of SERS [30]. EM can make the enhancement up to 1014 caused by the surface plasmon resonance [31,32] and CM provides additional enhancement by a charge transfer process [33–35]. Therefore, combination of the both two mechanisms is meaningful. In the work of Sun, the nanoporous surface for SERS substrate was fabricated in batches and at low costs by cross stacking superaligned carbon nanotube films [36]; Zhang et al. chose
Corresponding author. Correspondence to: C. Yang, Institute of Materials and Clean Energy, Shandong Normal University, Jinan 250014, PR China. E-mail addresses: [email protected] (B. Man), [email protected] (C. Yang).
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https://doi.org/10.1016/j.apsusc.2019.05.179 Received 15 March 2019; Received in revised form 27 April 2019; Accepted 15 May 2019 Available online 17 May 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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AgNPs of different size and morphology via magnetron sputtering or high-temperature annealing, and assembled them on the surface and sidewalls of the aligned carbon nanotube (CNT) arrays to get a highly sensitive stable SERS substrate [37,38]. In our experiment, the Raman enhancement factor (EF) as high as 4.88 × 1011 was obtained, which demonstrates that this substrate has a satisfactory enhancement effect. The numerical simulations also prove the superior performance of these nanohybrids.
2.4. Numerical analysis of the electric field distribution
2. Experimental section
2.5. The SERS experiments
2.1. Materials
Rhodamine 6G (R6G) molecules are used as the target molecules for the SERS experiments. R6G powder was dissolved in deionized water to form the different concentration solution. R6G sample was dried up before the SERS test. 15 randomly selected spots on the substrate are measured and averaged to obtain the Raman spectra at each concentration. Raman spectroscopy was collected using a Horiba HR evolution 800 Raman microscope system with 532 nm laser (0.48 mW) with a spot size of 1um.
The electric field properties of samples were numerically calculated by the commercial finite difference time domain (FDTD) method. The size, morphology and structure of the samples were extracted by reference to the scanning electron microscope (SEM) images. The laser with a wavelength of 532 nm was used as the incident laser here, which was perpendicularly incident the sample.
Acetone (CH3COCH3, 99.5%), alcohol (C2H6O, 99.7%), Rhodamine 6G (R6G), and silver nitrate (AgNO3) were purchased from local chemical plant. Polyvinylpyrrolidone (PVP, Mw = 55,000) was purchased from Sigma-Aldrich. All chemicals were used without further purification. 2.2. Assembly of AgNPs
3. Results and discussions
The AgNPs used here were obtained by reducing AgNO3, which was mainly based on the method introduced by Zhang et al. [39]. Typically, 0.25 g PVP and 0.05 g AgNO3 were dissolved into the 20 mL ethylene glycol. The mixture was heated at 135 °C for 2 h.
Suspended AgNPs/CNTs nanohybrids were designed (Fig. 2(a)) and formed (Fig. 2(b)) in our experiment, the electric field distribution (Fig. 2(c)–(d)) with the near and far-field coupling were simulated by using the FDTD simulation. It can be clearly seen that the AgNPs density on the CNT nets is significantly higher than that on the silicon substrate, which proves that the AgNPs are more tend to adhere to the CNTs. The CNTs were firm enough as the carriers to support metal nanoparticles. AgNPs adhered on the surface of CNTs densely and uniformly to generate strong localized surface plasmon resonance (LSPR). Besides, the specific nano-cavities formed by the 3D network structure enabled the incident laser to oscillate, which was beneficial for electrical and optical application. The periodic-distribution array as well as the organized distribution of CNTs and AgNPs all enable the substrates to be a uniform test platform. To analyze and explain the performance of the structure, the electric field distributions and absorption features are numerically simulated via the FDTD method. The diameter of CNTs and the size of AgNPs used in this simulation are respectively set as 2 nm and 80 nm, dense CNTs can be considered as a thin layer. For this structure, the density of the AgNPs is conveniently regulated. Moreover, as the structural density changes, its enhancement mechanism also changes. Therefore, we designed two models with different density for analysis. For the compact AgNPs/CNT/AgNPs structure, the gaps
2.3. Preparation of the suspended AgNPs/CNT nanohybrids The Preparation of the nanohybrids is shown in Fig. 1. Suspended CNTs were firstly synthesized according to Liu's work [40]. A 100 nm Pd array was lithographically patterned on a SiO2/Si wafer, and then a 400 nm Ag film was deposited on the array. After this procedure, a layer of CNTs was deposited on the Ag film surface by the chemical vapor deposition (CVD). The ferrocene/sulfur powder was flowed into the growth zone along with the mixed gas of 1000 sccm argon and 10 sccm methane at a temperature of 1100 °C. At a specified time, the CNTs would be deposited on the surface of substrate placed in the deposition zone at a temperature of 150 °C. During the deposition, the density of CNTs was proportional to deposition time. Followed by annealing at 960 °C, the Ag films became to melt and aggregate towards the Pd array. The CNTs were followed with the Ag migration to the two ends of the Pd columns. Finally, 3 μL AgNPs solution was dropped onto the suspended CNT substrate by a pipette and dried at 80 °C in a nitrogen atmosphere.
Fig. 1. The synthesized processes of the suspended AgNPs/CNT nanohybrids. 1078
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Fig. 2. (a) Sketch map of the three-dimensional nanohybrids model. (b) SEM image of the nanohybrids. (c, d) The simulated electric field distributions of dense AgNPs (near-field) and sparsely arranged AgNPs (far-field).
measured on the substrate after the addition of AgNPs is compared with the previous one (Fig. 3(h)). The Raman peak of the CNT can still be clearly seen in the figure, and the heights of these peaks are significantly higher than previous one, which proves that the CNTs still exist after the process of dropping AgNPs solution, and also confirmed the Raman enhancement induced by AgNPs. It is noteworthy that the ratio of the height of the D peak at 1350 cm−1 to the height of the G peak at 1580 cm−1 (ID/IG) is significantly increased, which generally means an increase in defects of the CNT surface. It may be caused by a large amount of AgNPs adhering to the CNT surface. For exploring optimal AgNPs concentration for the SERS application, 3 μL AgNPs solution was dropped with the different times. Fabricated nanohybrids were shown in Fig. 4(a)–(e) respectively. It can be clearly seen that the density of AgNPs increased significantly with the increasing of the number of deposition times. Moreover, the AgNPs could be effectively intercepted and assembled along the CNTs to form suspending three-dimensional “hot spots”. Besides the excellent toughness property of CNTs, both ends of these CNTs were also embedded in the outer Ag layer of Pd. These properties of CNTs ensured that a certain amount of AgNPs can be caught by the CNTs networks. At the same time, it also proved that the substrate was sturdy enough to avoid the fracture of CNTs during the Raman experiments. R6G molecules, which are the common probe molecules, were used here to analyze the performance of such SERS substrates. R6G solution with a concentration of 10−7 M was selected to explore the Raman enhancement of the suspended 3D AgNPs/CNTs substrates. Fig. 5(a) and (b) show the Raman spectra of R6G on five substrates with different AgNPs concentration, which shows that an trend from rise to decline of the SERS signal are obtained with the increasing of the dropping times. With the concentration of the AgNPs increasing, the Raman intensity of R6G increases significantly, which can be attributed to the increasing of the AgNPs “hot spots”. It should be mentioned that the incident laser can be oscillated on the nanogaps and the layers of the AgNPs. The additional coupling of the laser on the layers of the AgNPs, which is caused by the cavity of the suspended CNTs network, can greatly enhance the effects of LSPR, as shown in the Fig. 5(c). However, as the concentration of AgNPs is further increased, AgNPs becomes to be aggregated, the number of the “hot spots” is decreased, cavity is much smaller, and the 3D nanostructures becomes to be vanished, so that the
between adjacent AgNPs are ca. 2 nm, and for the sparse AgNPs/CNT/ AgNPs structure, the intervals are set ca. 30 nm. Moreover, in the latter situation, there is 150 nm of distance from the bi-level CNTs. The numerical data for refractive index of C and Ag are respectively obtained from the previous literature [41,42]. In this simulation, the perfectly matched layer boundary condition and periodic boundary condition are respectively used in z and x(y) directions, and the mesh sizes are set as 0.2 nm in all the directions. As shown in Fig. 2(c), the strong electric field is mainly distributed in the nanogap between AgNPs and AgNPs. In its top layer, the maximum E/E0 is as high as 40. For the case with the larger nanogaps (Fig. 2(d)), the AgNPs between different layers form plasma coupling due to the far-field effect. It can be seen from the results of our simulation that even in the sixth layer of AgNPs, the E/E0 is still as high as 2.8. Therefore, the suspended AgNPs/CNTs nanohybrids can be expected to suit various fields because it has both strong LSPR effect and plasma coupling effect. CNTs can be successfully suspended between Pd pillars, which could be clearly observed in Fig. 3(a) and (b). Raman spectra were collected from random ten regions across the entire substrate, which was displayed in Fig. 3(c). It can be seen clearly that the peaks intensity of CNTs at various points was quite close to each other, which demonstrated the excellent uniformity of the substrate. The slight difference on intensity of Raman peaks at different location was mainly due to the little difference of the CNT number. We chose the G peak of CNT at 1580 cm−1 to measure the Raman mapping, and it shows the relatively smooth and uniform color distribution with only a little dark region (Fig. 3(d)), which indicates the prepared CNTs array is highly homogenous. AgNPs with the diameter of ~75 nm (Fig. 3(e)) were chosen here to be loaded onto the suspended CNTs networks to achieve that: 1) the suspended CNTs networks will not be broken; 2) the electric field distribution can be easily controlled on the near-field or far-field coupling (shown in Fig. 2(c)–(d)). For the formation of the suspended AgNPs/ CNTs, 3 μL solution of chemically synthesized AgNPs (1.05 g/mL) was dropped directly onto the substrate. The substrates were then heated at 80 °C to consolidate the AgNPs onto the CNTs. The AgNPs solution should be added dropwise to form the suspended nanohybrids. So that, AgNPs were self-aggregated onto the surface of suspended CNTs successfully, which is shown in Fig. 3(f)–(g). The Raman spectrum 1079
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Fig. 3. (a), (b) The SEM images of the suspended CNTs. (c) Raman spectra of the CNTs array. (d) Raman mapping of the peak at 1580 cm−1 over 20 μm × 20 μm area. (e) SEM image of the AgNPs on the silicon sheet. (f), (g) SEM image of the different concentrations of AgNPs self-aggregated on the CNT surface. (h) Comparison of Raman spectra of AgNPs/CNT and CNT.
specific surface area of the substrate and forms more three-dimensional hot spots, so its Raman enhancement effect is still higher than that of the AgNPs on the silicon substrate. At the same time, when more AgNPs cover the Pd array (Fig. 4(f)), the intensity of the measured Raman peak is quite close to that on the silicon substrate, which also proves our analysis. To demonstrate the SERS sensitivity, SERS spectra of R6G molecules with various concentrations are collected in Fig. 6(a). When the concentration is as low as 10−15 M, the 613, 774, 1185, 1315, 1365, 1508 and 1650 cm−1 peaks can still be easily observed (inset of the Fig. 6(b)). The 613 cm−1 peaks were selected to express the linear relation of R6G. As shown in Fig. 6(b), the coefficient of determination is as high as 0.987. This result proves that the substrate shows good Raman enhancement and excellent linearity. Because the AgNPs/CNTs nanohybrids was mainly dealing with average SERS signals in the Raman experiment, the detailed
Raman intensity of R6G is decreased significantly. To prove the advantage of this hybrid structure of the suspended 3D AgNPs/CNTs substrates, the various enhancement mechanisms should be analyzed one by one. Highest enhancement obtained on the 3D AgNPs/CNT structure due to the electromagnetic enhancement of the AgNPs and the chemical enhancement of the CNTs and the adsorption of the molecules by the suspended structures. The Raman spectra of the AgNPs on the polished silicon planes are detected in the same condition (shown in Fig. 5(d)). Comparing the enhancement by measuring the intensity of the characteristic peaks at 613 cm−1 in the Raman spectrum of R6G, it can be seen that the enhancement effect of the suspended 3D AgNPs/ CNTs substrate is about three times of the AgNPs on polished silicon planes. When the AgNPs are added in excess, the suspended 3D nanostructures are covered (Fig. 4(e)), and the enhancement of the substrate is mainly provided by the electromagnetic enhancement of the flat AgNPs. However, the presence of the Pd column array increases the
Fig. 4. SEM images of the suspended CNT (a) and the suspended AgNPs/CNTs with the different dropping times ((b) once, (c) twice, (d) three times, (e) four times and (f) five times). 1080
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Fig. 5. (a) Raman spectra of R6G on the five substrates with the different dropping times. (b) Raman intensity of the R6G peaks at 613, 773 and 1508 cm−1 on the five different substrates. (c) The additional coupling of the laser on the layers of the AgNPs. (d) The Raman spectra of the R6G on the suspended 3D AgNPs/CNTs and the silicon substrates (inset: SEM image of the AgNPs on the polished silicon planes).
NSERS roughly equals to the ratio of R6G concentration. Therefore, we adopted the approximate value of CRS/CSERS as the value of NRS/NSERS to obtain estimated EF. In the experiment, the intensity of the characteristic peaks at 613 cm−1 in the Raman spectrum of R6G with the concentration of 10−15 M is about 80. It has been calculated that the intensity of R6G peaks at 613 cm−1 with 10−3 M on the pure SiO2 substrate is 149.6. Hence, the ISERS/IRS and the NSERS/NRS can be calculated 0.53 and 1012. The EF on the substrate can be estimated as 5.35 × 1011, which proves that our substrate has excellent Raman enhancement. Homogeneity and reproducibility are important for the practical
distribution of the EF on the substrate, or even its maximum value, is irrelevant. The SERS enhancement factors (EF) for R6G on the suspended 3D AgNPs/CNTs substrates was calculated according to the standard equation:
EF =
ISERS / NSERS IRS / NRS
Here, ISERS and IRS, according to the previous reports [43], represent the peaks intensities of SERS spectra and the normal Raman spectra respectively. NSERS and NRS respectively represent the corresponding numbers of molecules within the incident laser spot. The value of NRS/
Fig. 6. (a) Raman spectra of the R6G with various concentrations. (b) Linear relation of R6G peaks intensities as a function of concentration at 613 cm−1 (inset: the Raman spectrum of R6G with the concentration of 10−15 M). 1081
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Fig. 7. (a) The Raman spectra measured randomly at 15 different locations. (b) Intensity of the 613 cm−1 peaks as a function of position.
SERS application. The large-area uniformly distributed suspended 3D AgNPs/CNTs nanohybrids are considered to have excellent uniformity. The SERS spectra of R6G molecules at 10−9 M from 15 random spots were recorded in Fig. 7(a), the results showed that the peak value of corresponding position is greatly consistent, the intensities for each peak also fluctuate quite mildly. The 613 cm−1 peaks were selected to intuitive investigate the uniformity (Fig. 7(b)). The 613 cm−1 peaks intensity at all positions display a small fluctuation near the average value (black solid line). The fluctuation ranges are restricted in the region from −6.3% to +9.8%, which demonstrates an excellent uniformity of the Raman signals.
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