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The fabrication of a high-sensitivity surface-enhanced Raman spectra substrate using texturization and electroplating technology
Applied Surface Science 490 (2019) 109–116
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The fabrication of a high-sensitivity surface-enhanced Raman spectra substrate using texturization and electroplating technology
T
Shupeng Liua, , Xuetao Wanga, Songpo Zhanga, Xiaofeng Lua, Taihao Lib, Fufei Panga, ⁎ Zhenyi Chena, Na Chena, ⁎
a
Key Laboratory of Specialty Fiber Optics and Optical Access Networks, Joint International Research Laboratory of Specialty Fiber Optics and Advanced Communication, Shanghai Institute for Advanced Communication and Data Science, Institute of Biomedical Engineering, School of Communication and Information Engineering, Shanghai University, 333 Nanchen Road, Shanghai 200444, China b College of Medical Instruments, Shanghai University of Medicine & Health Sciences, Shanghai 201318, China
ARTICLE INFO
ABSTRACT
Keywords: SERS Substrate Texturization Electroplating Inverted pyramid arrays
A method for preparing surface-enhanced Raman scattering substrate with high sensitivity is presented in this study. The new type of substrate was fabricated by depositing silver nanoparticles, using electroplating, onto the surface of boron-doped single silicon with a texture of inverted pyramid arrays prepared by the technology of texturization. In order to evaluate the enhancement ability of the substrate, a test of Raman scattering with Rhodamine 6G (R6G) as probe molecules was performed and the results showed a high sensitivity, good uniformity and stability. Relay on the enhancement by the substrate, the Raman scattering signal of R6G with a high signal-to-noise ratio could be detected out at a rather low concentration level of 10−18 M. The performance of the substrate is also simulated and illustrated using the software COMSOL Multiphysics. The electric field is greatly enhanced on the surface of silicon wafer with inverted pyramid morphology, especially in pits. According to the electromagnetic enhancement theory, the calculated Raman enhancement factor is as high as 1011, which strongly supports the experimental data. This work may offer a novel and practical method for Raman spectroscopy application in trace analysis.
1. Introduction Surface-enhanced Raman scattering (SERS) spectroscopy has been considered one of the most powerful probing tools since its discovery in 1974 [1] due to its high sensitivity for trace analysis and even singlemolecule detection [2]. It has been reported that the SERS signals can be amplified by 105-106 when the target molecules adsorbed properly on the surface of some noble metals such as Au and Ag [3,4]. The mechanism of the enhancement is almost universally accepted to be the amplification of the local electromagnetic fields owing to the excitation of surface plasmonic resonance [5–7]. Therefore, the preparation of substrates interacting with metal nanoparticles (NPs) to produce high intense local electromagnetic (generally called “hotspot”) is extremely important. Many studies have devoted to the optimization of SERS substrates for high enhancement and for ease use [8–11]. In the past decades, the SERS-active substrates are well-designed and optimized in the aspects of structure and materials. In terms of spatial structure, the distribution of nanoparticles extends from one dimension (such as silver electrodes) [12] to three dimensions (such as
⁎
vertically aligned silicon nanowires) [13–15]. In terms of particles shape, additional structures such as nanometer-sized tips, edges, grooves or ridges were synthesized compared to spherical particles [16,17]. In terms of materials, materials for preparing substrates gradually expended from the noble metals to semiconductor materials (for example, graphene oxide and Titanium dioxide) [18,19] and even to biological materials [20,21]. Moreover, the approaches for the preparation of SERS-active substrates have also been developed from chemical reduction [22] to advanced nanofabrication technique, such as microlithographic techniques [23], electron-beam lithography (EBL) [24], focused ion beam (FIB) [25], and so on. Although a variety of SERS-active substrates have been made applying to trace-molecule detection, biomolecule analysis, material characterization and multiple other applications, there are still some factors prohibiting the range of SERS substrates being further expanded. For instance, it is not possible to control the hotspots of the colloidal NPs leading to the difficulty in reproducibility. Additionally, nanostructure with controllable interparticle nanogaps can be made with the assistance of the nanoscience and nanotechnology, but these complex preparation processes are too
Corresponding authors. E-mail addresses: [email protected] (S. Liu), [email protected] (N. Chen).
https://doi.org/10.1016/j.apsusc.2019.06.082 Received 10 March 2019; Received in revised form 13 May 2019; Accepted 8 June 2019 Available online 10 June 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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costly and not suitable for industrial production. Recently, many studies devoted to employing microstructure [26–29], particularly the porous silicon [30–32], with large surface area and unique structure for SERS substrates in attempting to get excellent performance. Therefore, we proposed an idea that the deposition of silver nanoparticles on the surface of silicon with randomly inverted pyramid arrays as an enhancement substrate. In this study, we developed a simple and efficient method to fabricate a cost-effective SERS-active substrate with a large area, high enhancement factor (EF), superior reproducibility, and excellent repeatability. First, we made inverted pyramids arrays on the surface of silicon wafers with the use of maskless Cu-NPs assisted anisotropic etching of crystalline silicon in Cu(NO3)2/HF/H2O2/H2O [33], which, on a larger scale, allows NPs attached to an inverted pyramid as a unit to exhibit periodic distribution. Then, different from the current widely used chemical reduction method for preparation of NPs, we directly deposited NPs on the surface of silicon by electroplating. This substrate performance was investigated with Rhodamine 6G (R6G) as a probe molecule. Compared with the classical method-Lee and Meisel's trisodium citrate reduction method-for preparation of Ag sol, it obtained lower detection limit. In addition, COMSOL Multiphysics was used to analyze the enhancement mechanism by simulation, and the theoretical calculation was in agreement with the experimental data.
2.2. Fabrication of SERS substrates
2. Experiment materials and methods
Silver colloid was synthesized referring to Lee and Meisel's trisodium citrate reduction method. Briefly, 1 ml of 0.1 M AgNO3 solution was added to 99 ml ultrapure water and heated to boiling. Subsequently, 1.8 ml of 1% trisodium citrate tribasic solution was added in the solution with rapid stirring. The mixture was then kept boiling gently till the color turned celadon. Finally, the prepared silver colloid was transferred into a centrifuge tube and centrifuged at 7800 rpm for 10 min and then the supernate was removed.
Fig. 1 schematically illustrates the main process for the synthesis of SERS substrates. The process for preparation of SERS substrate mainly divided into two steps: texturization and electroplating. First of all, a PSi (boron-doped single crystal silicon) wafer with (100) plane was rinsed with acetone, ethanol, and ultrapure water in sequence. Then, the rinsed PSi was immersed into a polypetrafluoroethylene container filled with mixed solution of 5 mM CuSO4, 4.6 M HF and 0.55 M H2O2. Thus, the inverted pyramid arrays were obtained when the wafer was etched for 15 min at 50 °C. Subsequently, as shown in Fig. 2, PSi and graphite rod, connected with the positive and negative pole of the DC source respectively, were inserted into the tank containing AgNO3 solution with the concentration of 0.025 M. The distance between PSi and graphite rod in tank is about 1 cm. The electroplating voltage is 5 V and the current is about 5 mA. After 60 s, high density Ag NPs with welldistribution were rapidly deposited on the inverted pyramid arrays of PSi. Afterwards, it can be clearly seen that a layer of silver-white coated on the surface of the PSi. Residual AgNO3 were removed using ultrapure water by washing at least three times and the fresh substrate was stored in low temperature environment such as refrigerator for further measurement. 2.3. Preparation of concentrated colloidal AgNPs
2.1. Materials Boron-doped crystalline silicon wafers (1-2Ωcm), were purchased from Kai Hua crystal silicon material Co. Ltd., Zhejiang, China. Silver nitrate(AgNO3, 0.103 M), trisodium citrate (Na3C6H5O7 · 2H2O), copper sulphate (CuSO4), hydrofluoric acid (HF), hydrogen peroxide (H2O2) and Rhodamine 6G (R6G) were supplied by the Key Laboratory of Specialtly Fiber Optics and Optical Access Networks, Shanghai University. All chemical reagents were of analytical grade and were used without further purification. Ultrapure water (18.3MΩ) was used for all solution preparation.
2.4. Preparation of flat PSi decorated with Ag-NPs In order to illustrate the performance of the substrate with inverted pyramid arrays, we prepared a PSi decorated with Ag NPs without any surface treatment (called flat PSi). The processes of preparing flat PSi
Fig. 1. Schematic illustration of substrates preparation process. 110
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Fig. 2. Configuration diagram of electroplating equipment used for deposition of Ag NPs on PSi.
are exactly same to that of the inverted pyramid substrate except for texturization.
about 1 μm, which shows a densely distributed. Fig. 3b is the microscopy image of PSi after electroplating taken by Raman spectrometer. The bottom of each inverted pyramid pit is approximately square and the top is sharp, just like a regular tetrahedron. Fig. 3c shows the SEM image of PSi after electroplating. In a large scale, the distribution of nanoparticles is uniform, which, to some extent, provide the necessary condition for reproducibility of SERS. As far as a single nanoparticle, the size is about 30–500 nm in diameter, the nanoparticles are arranged very closely, especially in inverted pyramid pit where gather more nanoparticles, so it makes a nanoparticle to form a number of gaps with the surrounding nanoparticles. In theory, this creates a condition for producing a high density hot spots area. In particular, it is pointed out that many edges or tips will be created naturally on the silicon wafer in the process of texturization, which may interact with nanoparticles to produces a very strong local electric field. If the substrate is cleaned by ultrasonic cleaner, the microstructure will be broken due to the highspeed vibration of the substrate. In view of this, although the ultrasonic cleaning machine is very effective in cleaning chemical containers and other items in peacetime, it is not recommended to use it to clean the substrate.
2.5. Preparation of samples for SERS measurement A series of different concentration of Rhodamine 6G (R6G) as probe molecules were prepared for measurement of SERS performance of substrates. 4.79 mg R6G powder was taken into a beaker, adding to some ultrapure water and stirring till dissolved. Then the R6G solution was poured into volumetric flask of 100 ml, diluted with ultrapure water to constant volume to make a R6G solution with concentration of 10−4 M. R6G solution were prepared with different concentration ranging from 10−5 to 10−20 M by the method of concentration gradient dilution. 2.6. Characterization and SERS measurement The surface morphology of the substrate was characterized by field emission scanning electron microscopy (SEM, COXEM EM-30 PLUS) under an acceleration voltage of 15.0 kV. Raman signal was collected with a laser confocal Raman spectrometer (Horbia LabRAM HR Evolutilon) with laser wavelength of 532, 633 and 785 nm. The simulation of electromagnetic enhancement mechanism of the substrate was used COMSOL Multiphysics in the wave optical mode.
3.2. Characterization of substrate performance For ease of explanation, the substrate prepared by texturation and electroplating is called substrate 1, without texturization called substrate 2, and the concentrated silver colloid is called substrate 3. A532nm laser with a power of 44 mW and gratings of 600 g mm−1 was used as the excitation light for measurements, and the filter of Raman spectrometer was set as 10%, with an integration time of 3 s and scan range from 400 to 1800 cm−1. Fig. 4 is a Raman spectrogram collected by several samples-dropping different concentrations of R6G solution on SERS substrates. The main characteristic peaks (612, 773, 1185, 1312, 1362, 1509 and
3. Results and discussion 3.1. Characterization of substrate morphology The SEM image of Psi after texturization (Fig. 3a) shows that inverted pyramids arrays with various sizes distribute randomly on the surface of silicon. The side lengthen of the entrance for the inverted pyramids is about 100–250 nm. The distance between the pyramids is 111
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(b) Microscopy image of the SERS substrate after electroplating
(a) SEM image of the SERS substrate after texturization
(c) SEM image of the SERS substrate after electroplating Fig. 3. (a) SEM image of the SERS substrate after texturization. (b) Microscopy image of the SERS substrate after electroplating. (c) SEM image of the SERS substrate after electroplating.
1649 cm−1) of R6G were all detected. Obviously, the highest peak is located at 520 cm−1, the characteristic peak of silicon, and its presence because of the use of PSi as substrate. From the Raman spectrum, three characteristics were observed: 1. The detection range is so wide that the detection limit is as low as 10−18 M; 2. Raman spectrum is pure with less noise interference and high signal-to noise ratio; 3. The proportionality between intensity of SERS and concentration of R6G is not significant, the change degree of signal intensity is far less than that of concentration. In a considerable large range, intensity-concentration curve has a flat bottom. It can be illustrated in Fig. 5 which is plotted by the average of several parallel data corresponding to the same sample. What worth to note is that a low-concentration R6G solution may has a higher intensity than that of high-concentration. And the reason is that the substrate 1 meets the condition for surface plasmonic resonance exactly in a certain preparation process, which results in a significant enhancement of Raman scattering. 0.1 ml of a 10−7 and 10−5 M R6G solution was dropped on the substrate 1 and 2, respectively. The Raman spectrum was shown in Fig. 6. Raman signal based on substrate 2 is very weak compared to fluorescence background. Except from the peak located at 612 and 774 cm−1, the rest was even completely overwhelmed by background
noise and almost impossible to identify. This is because the surface of silicon wafer is relatively flat, and the deposition of AgNPs is smooth, leading to the strong reflection of excitation light. Furthermore, hot spots only formed on the surface layer that the enhancement effect is poor. Although the concentration of R6G on the substrate 1 is lower, the signal intensity is much better than that of substrate 2. Not only does it reduces the fluorescence background, but also improves the signal-tonoise radio. Silver colloid, prepared by an aqueous reduction of silver nitrate with trisodium citrate according to Lee and Meisel's method, is a widely used SERS substrate for research on Raman spectroscopy [34,35]. Later, silver colloid was centrifuged to further aggregate, and it was reported that an ultrasensitive and facile SERS approach, a two-step centrifugation method, achieved a detection limit of 500 fM with phenformin hydrochloride and risperidone as acidic and alkaline analyte [36]. 0.1 ml of a 10−15 M R6G solution was dropped on the substrate 1 and 3, respectively. The Raman spectra showed the two substrates still detected nearly all the characteristic peaks of R6G at very low concentrations (Fig. 7). However, besides the characteristic peaks of R6G, there are also many cluttered peaks (844 cm−1, 955 cm−1, 1030 cm−1) in Raman signals collected by substrate 3. This is because the substrate 112
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Fig. 4. Raman signal intensity of different concentration R6G solution dipping to the SERS substrate. Red curve for a 10−12M R6G; Blue curve for a 10−13M R6G;Green curve for a 10−16M R6G; Sky-blue curve for a 10−18M R6G.
Fig. 5. The curve of signal intensity as a function of concentration of R6G.
3 is prepared by chemical reduction method and the chemical reaction is incomplete so as to that there are some residues of trisodium citrate. In Fig. 7, there are non-R6G Raman peaks such as 844 cm−1 and 955 cm−1, which are the characteristic peaks of trisodium citrate. In addition, since the substrate 3 is liquid, the impurities of the container are liable to cause secondary pollution to the substrate 3, which can lead to a sharp deterioration of the Raman spectrum, especially in ultralow concentration detection. On the contrary, since exotic substances have not been introduced in the preparation process of substrate 1, there was no interference to the analyte except noise. For example, the peak of 1030 cm−1 in Fig. 7 is assigned to phenylalanine of collagen. The emergence of biological Raman peak is the result of contamination of containers by microorganisms. To some extent, the excitation and reception of Raman spectra can be regarded as a communication system. Here, the laser is the local oscillation signal, with the molecular vibration being the baseband
signal, and Raman scattering is the modulation of the baseband signal to the local oscillation signal. Raman scattering signal is received through the channel-Raman spectrometer-by the receiver, namely Charge Coupled Device (CCD). Of course, the signal will be superS imposed with noise. According to Shannon formula, C = Blog2 1 + N , the increase of signal-to-noise ratio will increase the channel capacity, and the amount of information reflects the intensity of the signal in Raman spectrum, I = ct (‘I’ is the amount of information, and ‘t’ is the acquisition time). If the same signal intensity is required, when the signal-to-noise ratio increases, the acquisition time can be reduced. 0.1 ml of a 10−7 M R6G solution was dropped on the substrate 1, and acquisition time was set by 3 s and 10 s. In theory, the signal intensity of acquisition time of 10 s should be three times that of 3 s. As shown in Fig. 8, the former with intensity about 27,000 is three times as strong as the latter with intensity about 9000.
(
113
)
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Fig. 6. the compare between 10−5 M R6G on substrate 2 and 10−7 M R6G on substrate 1.
Fig. 8. Raman spectra with different acquisition times.
between the particles and the silicon. In addition, the simple nanoparticle dimer hotspot is only in the polarization direction of the incident light, while the inverted pyramid can change the polarization direction of the scattered light, which makes the strong hotspot also exist in the direction perpendicular to the polarization direction of the incident light. We estimate the maximum of the fourth power of the electric field around hot spots as EF, according to the widely accepted mechanism of electromagnetic enhancement [37], EF = (Eloc/E0)4, where Eloc is the local electric field and E0 is the incident electric field. Therefore, according to our model simulation data, the maximum enhancement factor is 1.8 × 1011. It must be pointed out that for the complex system such as inverted pyramid array substrate, the simulation results can only explain roughly the surface enhanced Raman scattering effect. There must be a certain gap with the actual situation, whether in theory or in modeling. Moreover, conservatively, the gap of nanoparticles was set a bit larger in modeling, and it can be further enhanced if the gap is reduced in fact. The advantages of the method for preparing inverted pyramid substrate can be discussed simply. The preparation of substrate by nano-lithography technology requires very expensive machinery and equipment, which is not conducive to promotion. The preparation method of inverted pyramid substrate is simple, convenient and cheap using the common equipment such as DC source. The self-assembly method for preparing substrate is needed to be solidified for a long time, which is very time-consuming. However, the substrate of the inverted pyramid can be etched in batches for later use, and electroplating only takes 1 min, which is very fast and convenient. As for the preparation of substrate by chemical reduction method, it is inevitable that incomplete reaction will result in residual reactants, and Raman peaks of reactants will interfere with Raman peaks of analytes. For example, the residual trisodium citrate in the silver sol in Fig. 7 will appear in the Raman peak of R6G. Whereas, there is no such problem in the preparation of the inverted pyramid because it does not involve any organic matter.
Fig. 7. Signal-to-noise ratio comparison of Raman spectra collected from two substrates at low R6G concentration.
3.3. Simulation for model of SERS substrate In order to study the enhancement mechanism of inverted pyramid arrays substrate, COMSOL Multiphysics was utilized to model and calculate the spatial distributions of the electromagnetic field intensity for it. Tetrahedral grooves on cuboids were used to simulate the inverted pyramid pits on silicon wafers and spherical particles were used to simulate the deposition of silver nanoparticles. According to the SEM image of the substrate, the mosaic model of small spheres and large spheres represents the randomness and contingency of silver nanoparticles deposition. The incident light (532 nm) with x-polarization propagated along the –z direction. The magnitude of the incident field is 1 V/m. As can be seen from Fig. 9(c) and (d), although the nanoparticles have the same placement structure, they are nearly twice as large in the inverted pyramid as when they exist alone. Fig. 9(a) is a common silver nanoparticle dimer. Compared with Fig. 9(b), it seems that the electric field can be further enhanced when small particles in the gap between large particles. From Fig. 9(a) to (d), we find that the electric field is not only getting stronger and stronger, but also the hot spots are not limited to the particles. There is also an electric field
4. Conclusion In this study, we fabricated a SERS-active substrate combined the technology of texturization and electroplating using a simple and lowcost method. Based on the SERS results, we can obtain SERS signals with high sensitivity using R6G as a probe. The enhancement ability and performance of the substrate showed in a Raman scattering test were almost agreed with the simulation by COMSOL Multiphysics. 114
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Fig. 9. (a) The x-z view of the electric field distribution of Ag dimers (70 nm in radius, 4 nm gap size in air); (b) the x-z view of the electric field distribution of small nanoparticle embedded between two large nanoparticles (70 nm in radius of large particle, 30 nm in radius of small particle, 4 nm gap size in air); (c) the x-z view of the electric field distribution of nanoparticles oligomer (70 nm in radius of large particle, 30 nm in radius of small particle, 4 nm gap size in air); (d) the x-z view of the electric field distribution of nanoparticles deposited on the inverted pyramid pit (70 nm in radius of large particle, 30 nm in radius of small particle, 4 nm gap size in air).
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Full length article
The fabrication of a high-sensitivity surface-enhanced Raman spectra substrate using texturization and electroplating technology
T
Shupeng Liua, , Xuetao Wanga, Songpo Zhanga, Xiaofeng Lua, Taihao Lib, Fufei Panga, ⁎ Zhenyi Chena, Na Chena, ⁎
a
Key Laboratory of Specialty Fiber Optics and Optical Access Networks, Joint International Research Laboratory of Specialty Fiber Optics and Advanced Communication, Shanghai Institute for Advanced Communication and Data Science, Institute of Biomedical Engineering, School of Communication and Information Engineering, Shanghai University, 333 Nanchen Road, Shanghai 200444, China b College of Medical Instruments, Shanghai University of Medicine & Health Sciences, Shanghai 201318, China
ARTICLE INFO
ABSTRACT
Keywords: SERS Substrate Texturization Electroplating Inverted pyramid arrays
A method for preparing surface-enhanced Raman scattering substrate with high sensitivity is presented in this study. The new type of substrate was fabricated by depositing silver nanoparticles, using electroplating, onto the surface of boron-doped single silicon with a texture of inverted pyramid arrays prepared by the technology of texturization. In order to evaluate the enhancement ability of the substrate, a test of Raman scattering with Rhodamine 6G (R6G) as probe molecules was performed and the results showed a high sensitivity, good uniformity and stability. Relay on the enhancement by the substrate, the Raman scattering signal of R6G with a high signal-to-noise ratio could be detected out at a rather low concentration level of 10−18 M. The performance of the substrate is also simulated and illustrated using the software COMSOL Multiphysics. The electric field is greatly enhanced on the surface of silicon wafer with inverted pyramid morphology, especially in pits. According to the electromagnetic enhancement theory, the calculated Raman enhancement factor is as high as 1011, which strongly supports the experimental data. This work may offer a novel and practical method for Raman spectroscopy application in trace analysis.
1. Introduction Surface-enhanced Raman scattering (SERS) spectroscopy has been considered one of the most powerful probing tools since its discovery in 1974 [1] due to its high sensitivity for trace analysis and even singlemolecule detection [2]. It has been reported that the SERS signals can be amplified by 105-106 when the target molecules adsorbed properly on the surface of some noble metals such as Au and Ag [3,4]. The mechanism of the enhancement is almost universally accepted to be the amplification of the local electromagnetic fields owing to the excitation of surface plasmonic resonance [5–7]. Therefore, the preparation of substrates interacting with metal nanoparticles (NPs) to produce high intense local electromagnetic (generally called “hotspot”) is extremely important. Many studies have devoted to the optimization of SERS substrates for high enhancement and for ease use [8–11]. In the past decades, the SERS-active substrates are well-designed and optimized in the aspects of structure and materials. In terms of spatial structure, the distribution of nanoparticles extends from one dimension (such as silver electrodes) [12] to three dimensions (such as
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vertically aligned silicon nanowires) [13–15]. In terms of particles shape, additional structures such as nanometer-sized tips, edges, grooves or ridges were synthesized compared to spherical particles [16,17]. In terms of materials, materials for preparing substrates gradually expended from the noble metals to semiconductor materials (for example, graphene oxide and Titanium dioxide) [18,19] and even to biological materials [20,21]. Moreover, the approaches for the preparation of SERS-active substrates have also been developed from chemical reduction [22] to advanced nanofabrication technique, such as microlithographic techniques [23], electron-beam lithography (EBL) [24], focused ion beam (FIB) [25], and so on. Although a variety of SERS-active substrates have been made applying to trace-molecule detection, biomolecule analysis, material characterization and multiple other applications, there are still some factors prohibiting the range of SERS substrates being further expanded. For instance, it is not possible to control the hotspots of the colloidal NPs leading to the difficulty in reproducibility. Additionally, nanostructure with controllable interparticle nanogaps can be made with the assistance of the nanoscience and nanotechnology, but these complex preparation processes are too
Corresponding authors. E-mail addresses: [email protected] (S. Liu), [email protected] (N. Chen).
https://doi.org/10.1016/j.apsusc.2019.06.082 Received 10 March 2019; Received in revised form 13 May 2019; Accepted 8 June 2019 Available online 10 June 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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costly and not suitable for industrial production. Recently, many studies devoted to employing microstructure [26–29], particularly the porous silicon [30–32], with large surface area and unique structure for SERS substrates in attempting to get excellent performance. Therefore, we proposed an idea that the deposition of silver nanoparticles on the surface of silicon with randomly inverted pyramid arrays as an enhancement substrate. In this study, we developed a simple and efficient method to fabricate a cost-effective SERS-active substrate with a large area, high enhancement factor (EF), superior reproducibility, and excellent repeatability. First, we made inverted pyramids arrays on the surface of silicon wafers with the use of maskless Cu-NPs assisted anisotropic etching of crystalline silicon in Cu(NO3)2/HF/H2O2/H2O [33], which, on a larger scale, allows NPs attached to an inverted pyramid as a unit to exhibit periodic distribution. Then, different from the current widely used chemical reduction method for preparation of NPs, we directly deposited NPs on the surface of silicon by electroplating. This substrate performance was investigated with Rhodamine 6G (R6G) as a probe molecule. Compared with the classical method-Lee and Meisel's trisodium citrate reduction method-for preparation of Ag sol, it obtained lower detection limit. In addition, COMSOL Multiphysics was used to analyze the enhancement mechanism by simulation, and the theoretical calculation was in agreement with the experimental data.
2.2. Fabrication of SERS substrates
2. Experiment materials and methods
Silver colloid was synthesized referring to Lee and Meisel's trisodium citrate reduction method. Briefly, 1 ml of 0.1 M AgNO3 solution was added to 99 ml ultrapure water and heated to boiling. Subsequently, 1.8 ml of 1% trisodium citrate tribasic solution was added in the solution with rapid stirring. The mixture was then kept boiling gently till the color turned celadon. Finally, the prepared silver colloid was transferred into a centrifuge tube and centrifuged at 7800 rpm for 10 min and then the supernate was removed.
Fig. 1 schematically illustrates the main process for the synthesis of SERS substrates. The process for preparation of SERS substrate mainly divided into two steps: texturization and electroplating. First of all, a PSi (boron-doped single crystal silicon) wafer with (100) plane was rinsed with acetone, ethanol, and ultrapure water in sequence. Then, the rinsed PSi was immersed into a polypetrafluoroethylene container filled with mixed solution of 5 mM CuSO4, 4.6 M HF and 0.55 M H2O2. Thus, the inverted pyramid arrays were obtained when the wafer was etched for 15 min at 50 °C. Subsequently, as shown in Fig. 2, PSi and graphite rod, connected with the positive and negative pole of the DC source respectively, were inserted into the tank containing AgNO3 solution with the concentration of 0.025 M. The distance between PSi and graphite rod in tank is about 1 cm. The electroplating voltage is 5 V and the current is about 5 mA. After 60 s, high density Ag NPs with welldistribution were rapidly deposited on the inverted pyramid arrays of PSi. Afterwards, it can be clearly seen that a layer of silver-white coated on the surface of the PSi. Residual AgNO3 were removed using ultrapure water by washing at least three times and the fresh substrate was stored in low temperature environment such as refrigerator for further measurement. 2.3. Preparation of concentrated colloidal AgNPs
2.1. Materials Boron-doped crystalline silicon wafers (1-2Ωcm), were purchased from Kai Hua crystal silicon material Co. Ltd., Zhejiang, China. Silver nitrate(AgNO3, 0.103 M), trisodium citrate (Na3C6H5O7 · 2H2O), copper sulphate (CuSO4), hydrofluoric acid (HF), hydrogen peroxide (H2O2) and Rhodamine 6G (R6G) were supplied by the Key Laboratory of Specialtly Fiber Optics and Optical Access Networks, Shanghai University. All chemical reagents were of analytical grade and were used without further purification. Ultrapure water (18.3MΩ) was used for all solution preparation.
2.4. Preparation of flat PSi decorated with Ag-NPs In order to illustrate the performance of the substrate with inverted pyramid arrays, we prepared a PSi decorated with Ag NPs without any surface treatment (called flat PSi). The processes of preparing flat PSi
Fig. 1. Schematic illustration of substrates preparation process. 110
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Fig. 2. Configuration diagram of electroplating equipment used for deposition of Ag NPs on PSi.
are exactly same to that of the inverted pyramid substrate except for texturization.
about 1 μm, which shows a densely distributed. Fig. 3b is the microscopy image of PSi after electroplating taken by Raman spectrometer. The bottom of each inverted pyramid pit is approximately square and the top is sharp, just like a regular tetrahedron. Fig. 3c shows the SEM image of PSi after electroplating. In a large scale, the distribution of nanoparticles is uniform, which, to some extent, provide the necessary condition for reproducibility of SERS. As far as a single nanoparticle, the size is about 30–500 nm in diameter, the nanoparticles are arranged very closely, especially in inverted pyramid pit where gather more nanoparticles, so it makes a nanoparticle to form a number of gaps with the surrounding nanoparticles. In theory, this creates a condition for producing a high density hot spots area. In particular, it is pointed out that many edges or tips will be created naturally on the silicon wafer in the process of texturization, which may interact with nanoparticles to produces a very strong local electric field. If the substrate is cleaned by ultrasonic cleaner, the microstructure will be broken due to the highspeed vibration of the substrate. In view of this, although the ultrasonic cleaning machine is very effective in cleaning chemical containers and other items in peacetime, it is not recommended to use it to clean the substrate.
2.5. Preparation of samples for SERS measurement A series of different concentration of Rhodamine 6G (R6G) as probe molecules were prepared for measurement of SERS performance of substrates. 4.79 mg R6G powder was taken into a beaker, adding to some ultrapure water and stirring till dissolved. Then the R6G solution was poured into volumetric flask of 100 ml, diluted with ultrapure water to constant volume to make a R6G solution with concentration of 10−4 M. R6G solution were prepared with different concentration ranging from 10−5 to 10−20 M by the method of concentration gradient dilution. 2.6. Characterization and SERS measurement The surface morphology of the substrate was characterized by field emission scanning electron microscopy (SEM, COXEM EM-30 PLUS) under an acceleration voltage of 15.0 kV. Raman signal was collected with a laser confocal Raman spectrometer (Horbia LabRAM HR Evolutilon) with laser wavelength of 532, 633 and 785 nm. The simulation of electromagnetic enhancement mechanism of the substrate was used COMSOL Multiphysics in the wave optical mode.
3.2. Characterization of substrate performance For ease of explanation, the substrate prepared by texturation and electroplating is called substrate 1, without texturization called substrate 2, and the concentrated silver colloid is called substrate 3. A532nm laser with a power of 44 mW and gratings of 600 g mm−1 was used as the excitation light for measurements, and the filter of Raman spectrometer was set as 10%, with an integration time of 3 s and scan range from 400 to 1800 cm−1. Fig. 4 is a Raman spectrogram collected by several samples-dropping different concentrations of R6G solution on SERS substrates. The main characteristic peaks (612, 773, 1185, 1312, 1362, 1509 and
3. Results and discussion 3.1. Characterization of substrate morphology The SEM image of Psi after texturization (Fig. 3a) shows that inverted pyramids arrays with various sizes distribute randomly on the surface of silicon. The side lengthen of the entrance for the inverted pyramids is about 100–250 nm. The distance between the pyramids is 111
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(b) Microscopy image of the SERS substrate after electroplating
(a) SEM image of the SERS substrate after texturization
(c) SEM image of the SERS substrate after electroplating Fig. 3. (a) SEM image of the SERS substrate after texturization. (b) Microscopy image of the SERS substrate after electroplating. (c) SEM image of the SERS substrate after electroplating.
1649 cm−1) of R6G were all detected. Obviously, the highest peak is located at 520 cm−1, the characteristic peak of silicon, and its presence because of the use of PSi as substrate. From the Raman spectrum, three characteristics were observed: 1. The detection range is so wide that the detection limit is as low as 10−18 M; 2. Raman spectrum is pure with less noise interference and high signal-to noise ratio; 3. The proportionality between intensity of SERS and concentration of R6G is not significant, the change degree of signal intensity is far less than that of concentration. In a considerable large range, intensity-concentration curve has a flat bottom. It can be illustrated in Fig. 5 which is plotted by the average of several parallel data corresponding to the same sample. What worth to note is that a low-concentration R6G solution may has a higher intensity than that of high-concentration. And the reason is that the substrate 1 meets the condition for surface plasmonic resonance exactly in a certain preparation process, which results in a significant enhancement of Raman scattering. 0.1 ml of a 10−7 and 10−5 M R6G solution was dropped on the substrate 1 and 2, respectively. The Raman spectrum was shown in Fig. 6. Raman signal based on substrate 2 is very weak compared to fluorescence background. Except from the peak located at 612 and 774 cm−1, the rest was even completely overwhelmed by background
noise and almost impossible to identify. This is because the surface of silicon wafer is relatively flat, and the deposition of AgNPs is smooth, leading to the strong reflection of excitation light. Furthermore, hot spots only formed on the surface layer that the enhancement effect is poor. Although the concentration of R6G on the substrate 1 is lower, the signal intensity is much better than that of substrate 2. Not only does it reduces the fluorescence background, but also improves the signal-tonoise radio. Silver colloid, prepared by an aqueous reduction of silver nitrate with trisodium citrate according to Lee and Meisel's method, is a widely used SERS substrate for research on Raman spectroscopy [34,35]. Later, silver colloid was centrifuged to further aggregate, and it was reported that an ultrasensitive and facile SERS approach, a two-step centrifugation method, achieved a detection limit of 500 fM with phenformin hydrochloride and risperidone as acidic and alkaline analyte [36]. 0.1 ml of a 10−15 M R6G solution was dropped on the substrate 1 and 3, respectively. The Raman spectra showed the two substrates still detected nearly all the characteristic peaks of R6G at very low concentrations (Fig. 7). However, besides the characteristic peaks of R6G, there are also many cluttered peaks (844 cm−1, 955 cm−1, 1030 cm−1) in Raman signals collected by substrate 3. This is because the substrate 112
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Fig. 4. Raman signal intensity of different concentration R6G solution dipping to the SERS substrate. Red curve for a 10−12M R6G; Blue curve for a 10−13M R6G;Green curve for a 10−16M R6G; Sky-blue curve for a 10−18M R6G.
Fig. 5. The curve of signal intensity as a function of concentration of R6G.
3 is prepared by chemical reduction method and the chemical reaction is incomplete so as to that there are some residues of trisodium citrate. In Fig. 7, there are non-R6G Raman peaks such as 844 cm−1 and 955 cm−1, which are the characteristic peaks of trisodium citrate. In addition, since the substrate 3 is liquid, the impurities of the container are liable to cause secondary pollution to the substrate 3, which can lead to a sharp deterioration of the Raman spectrum, especially in ultralow concentration detection. On the contrary, since exotic substances have not been introduced in the preparation process of substrate 1, there was no interference to the analyte except noise. For example, the peak of 1030 cm−1 in Fig. 7 is assigned to phenylalanine of collagen. The emergence of biological Raman peak is the result of contamination of containers by microorganisms. To some extent, the excitation and reception of Raman spectra can be regarded as a communication system. Here, the laser is the local oscillation signal, with the molecular vibration being the baseband
signal, and Raman scattering is the modulation of the baseband signal to the local oscillation signal. Raman scattering signal is received through the channel-Raman spectrometer-by the receiver, namely Charge Coupled Device (CCD). Of course, the signal will be superS imposed with noise. According to Shannon formula, C = Blog2 1 + N , the increase of signal-to-noise ratio will increase the channel capacity, and the amount of information reflects the intensity of the signal in Raman spectrum, I = ct (‘I’ is the amount of information, and ‘t’ is the acquisition time). If the same signal intensity is required, when the signal-to-noise ratio increases, the acquisition time can be reduced. 0.1 ml of a 10−7 M R6G solution was dropped on the substrate 1, and acquisition time was set by 3 s and 10 s. In theory, the signal intensity of acquisition time of 10 s should be three times that of 3 s. As shown in Fig. 8, the former with intensity about 27,000 is three times as strong as the latter with intensity about 9000.
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Fig. 6. the compare between 10−5 M R6G on substrate 2 and 10−7 M R6G on substrate 1.
Fig. 8. Raman spectra with different acquisition times.
between the particles and the silicon. In addition, the simple nanoparticle dimer hotspot is only in the polarization direction of the incident light, while the inverted pyramid can change the polarization direction of the scattered light, which makes the strong hotspot also exist in the direction perpendicular to the polarization direction of the incident light. We estimate the maximum of the fourth power of the electric field around hot spots as EF, according to the widely accepted mechanism of electromagnetic enhancement [37], EF = (Eloc/E0)4, where Eloc is the local electric field and E0 is the incident electric field. Therefore, according to our model simulation data, the maximum enhancement factor is 1.8 × 1011. It must be pointed out that for the complex system such as inverted pyramid array substrate, the simulation results can only explain roughly the surface enhanced Raman scattering effect. There must be a certain gap with the actual situation, whether in theory or in modeling. Moreover, conservatively, the gap of nanoparticles was set a bit larger in modeling, and it can be further enhanced if the gap is reduced in fact. The advantages of the method for preparing inverted pyramid substrate can be discussed simply. The preparation of substrate by nano-lithography technology requires very expensive machinery and equipment, which is not conducive to promotion. The preparation method of inverted pyramid substrate is simple, convenient and cheap using the common equipment such as DC source. The self-assembly method for preparing substrate is needed to be solidified for a long time, which is very time-consuming. However, the substrate of the inverted pyramid can be etched in batches for later use, and electroplating only takes 1 min, which is very fast and convenient. As for the preparation of substrate by chemical reduction method, it is inevitable that incomplete reaction will result in residual reactants, and Raman peaks of reactants will interfere with Raman peaks of analytes. For example, the residual trisodium citrate in the silver sol in Fig. 7 will appear in the Raman peak of R6G. Whereas, there is no such problem in the preparation of the inverted pyramid because it does not involve any organic matter.
Fig. 7. Signal-to-noise ratio comparison of Raman spectra collected from two substrates at low R6G concentration.
3.3. Simulation for model of SERS substrate In order to study the enhancement mechanism of inverted pyramid arrays substrate, COMSOL Multiphysics was utilized to model and calculate the spatial distributions of the electromagnetic field intensity for it. Tetrahedral grooves on cuboids were used to simulate the inverted pyramid pits on silicon wafers and spherical particles were used to simulate the deposition of silver nanoparticles. According to the SEM image of the substrate, the mosaic model of small spheres and large spheres represents the randomness and contingency of silver nanoparticles deposition. The incident light (532 nm) with x-polarization propagated along the –z direction. The magnitude of the incident field is 1 V/m. As can be seen from Fig. 9(c) and (d), although the nanoparticles have the same placement structure, they are nearly twice as large in the inverted pyramid as when they exist alone. Fig. 9(a) is a common silver nanoparticle dimer. Compared with Fig. 9(b), it seems that the electric field can be further enhanced when small particles in the gap between large particles. From Fig. 9(a) to (d), we find that the electric field is not only getting stronger and stronger, but also the hot spots are not limited to the particles. There is also an electric field
4. Conclusion In this study, we fabricated a SERS-active substrate combined the technology of texturization and electroplating using a simple and lowcost method. Based on the SERS results, we can obtain SERS signals with high sensitivity using R6G as a probe. The enhancement ability and performance of the substrate showed in a Raman scattering test were almost agreed with the simulation by COMSOL Multiphysics. 114
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Fig. 9. (a) The x-z view of the electric field distribution of Ag dimers (70 nm in radius, 4 nm gap size in air); (b) the x-z view of the electric field distribution of small nanoparticle embedded between two large nanoparticles (70 nm in radius of large particle, 30 nm in radius of small particle, 4 nm gap size in air); (c) the x-z view of the electric field distribution of nanoparticles oligomer (70 nm in radius of large particle, 30 nm in radius of small particle, 4 nm gap size in air); (d) the x-z view of the electric field distribution of nanoparticles deposited on the inverted pyramid pit (70 nm in radius of large particle, 30 nm in radius of small particle, 4 nm gap size in air).
Funding information
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