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Ultrasensitive SERS detection of rhodamine 6G and p-nitrophenol based on electrochemically roughened nano-Au film
Journal Pre-proof Ultrasensitive SERS detection of rhodamine 6G and p-nitrophenol based on electrochemically roughened nano-Au film Jiangcai Wang, Cuicui Qiu, Xijiao Mu, Hua Pang, Xinchun Chen, Dameng Liu PII:
S0039-9140(19)31264-0
DOI:
https://doi.org/10.1016/j.talanta.2019.120631
Reference:
TAL 120631
To appear in:
Talanta
Received Date: 11 September 2019 Revised Date:
30 November 2019
Accepted Date: 7 December 2019
Please cite this article as: J. Wang, C. Qiu, X. Mu, H. Pang, X. Chen, D. Liu, Ultrasensitive SERS detection of rhodamine 6G and p-nitrophenol based on electrochemically roughened nano-Au film, Talanta (2020), doi: https://doi.org/10.1016/j.talanta.2019.120631. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Ultrasensitive SERS detection of rhodamine 6G and p-nitrophenol based on electrochemically roughened nano-Au film
Jiangcai Wanga, Cuicui Qiu*a,b, Xijiao Mu,c Hua Panga, Xinchun Chena, Dameng Liu∗a,b
a
State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
b
Tianjin Research Institute for Advanced Equipment, Tsinghua University, Tianjin
300300, China c
Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface
Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, China ∗
Corresponding author:
Cuicui Qiu, email: [email protected] Dameng Liu, Tel: 86-10-62797646; email: [email protected]
Abstract Quantitative analysis of organic pollutants in environmental water is an important issue for ecological environment and human health. In this paper, the quantitative analysis of rhodamine 6G (R6G) and p-nitrophenol (PNP) is performed by the surface enhanced Raman scattering (SERS) technology. The enhancement of Raman signals is achieved on the surface of an electrochemically roughened nano-Au film. The SERS performance depends on the microstructure of roughened nano-Au films, which is affected by the thickness of Au films and electrochemical roughening parameters. The structure-dependence of SERS performance is validated by finite element simulation of local electromagnetic field distribution. An obvious SERS effect of R6G with an enhancement factor of 108 is obtained on the roughened nano-Au film. A sensitive SERS detection of R6G with a linear range of 10-9-10-5 M and a detection limit of 10-11 M is realized. Moreover, a wide linear range of 10-9-10-3 M is obtained for the detection of PNP. The roughened nano-Au film is an effective substrate for the SERS detection of organic pollutants with high reproducibility and good stability. Therefore, the electrochemical technology in this study is expected to be a very promising method for the fabrication of high-performance SERS substrate.
Keywords: SERS detection; organic pollutants; electrochemical roughening; nano-Au
1. Introduction In recent years, as the water pollution is getting more serious, the number and type of pollutants in environmental water are increasing. The organics (like dye, phenols, pesticides, etc.) with high toxicity are common pollutants in the water environment, which cause a severe threat to human health and ecological environment [1-3]. The organic pollutants usually come from manufacturing industry, such as pesticides, pharmaceuticals, synthetic dyes, explosives, etc. Once the organic pollutants are exposed in the environment, the potential risks of pathopoiesis and carcinogenesis cannot be ignored [4]. The detection of organic pollutants has always been an important issue in the field of environmental protection. The urgent demand for the detection of organic pollutants has stimulated development of analytical techniques. Various detection techniques have been reported in the past decades, including spectroscopic techniques (like fluorescence spectroscopy [5], infrared spectroscopy [6], Raman spectroscopy [7] and UV-vis Spectroscopy [8]), electroanalytical techniques [9], mass-spectrometric techniques [10], etc. As a powerful detection method, Raman spectroscopy has been widely applied in the fields of material science, chemistry, biological and biomedical science. However, practical applications of Raman spectroscopy are largely limited due to its low signal intensity. Therefore, the enhancement of Raman signals is vital for the sensitive detection. Surface enhanced Raman scattering (SERS) technology with a strong Raman signal has been widely applied in ultra-sensitive chemical and biological analysis [11,
12]. The Raman signals of adsorbed molecules on the substrate surface can be significantly enhanced approximately 108-1011 due to the electromagnetic (EM) enhancement effect [13]. The EM enhancement effect plays a decisive role in the SERS phenomenon, which benefits from localized surface plasmons resonance (LSPR) of the metal nanoparticles [14, 15]. The EM enhancement mainly depends on the type and microstructure of substrate materials. The SERS substrate materials have been focused on Ag [16, 17], Au [18, 19], Cu [20, 21] and Ni [22], which own the suitable dielectric constant and strong surface plasmon band. It is well known that nanostructured Ag shows the strongest SERS effect. However, Ag surface is susceptible to oxidation/denaturation in the air/solutions. Thus, the nanostructured Au can become a promising alternative material due to its superior stability and strong plasmon band [23, 24]. The nanostructured Au has widely used in the fields of environment, biology, medicine and etc. It has been reported that the nanostructured Au with a large surface area can effectively improve the sensitivity of the SERS signals due to the increase of hot spots [25, 26]. Recently, a lot of works have focused on the SERS detection of organic pollutant in environment based on the nanostructured Au, like dye [27], pesticide residues [28], bisphenol A [29], persistent organic pollutant [30], cannabinol [31] and etc. As a major class of organic pollutants, phenolic compounds with high toxicity and persistence have attracted great attention in chemical and environmental fields. However, the direct SERS detection of phenolic compounds was limited on the metal nanomaterials surface due to a very weak Raman signal at low concentrations [32]. A
variety of nanostructured Au are designed to enhance the SERS performance by different preparation techniques (like hydrothermal method [33], chemical vapor deposition [34], and all femtosecond laser processing technique [35], etc.). However, some preparation techniques is too complicated and time-consuming for the practical application. Therefore, a facile and low-cost method is necessary for the preparation of high-performance SERS substrate materials. Considering that electrochemical technology is a facile method to fabricate nanomaterials, in this paper, a potential-step technology was used to prepare the high-performance nano-Au SERS substrate. During the electrochemical roughening process, the effect of the thickness of Au film and electrochemical parameters (like oxidation potentials and cyclic numbers) on the microstructure of roughened nano-Au films was investigated. Moreover, the microstructure of roughened nano-Au films had a significant effect on the SERS performance of R6G analyte molecules, which was validated by finite element simulation. Furthermore, SERS detection performance of R6G and p-nitrophenol (PNP) on the electrochemically roughened nano-Au was evaluated. 2. Experimental 2.1 Materials and regents R6G (AR grade) was purchased from Shanghai Macklin Biochemical Co., Ltd. PNP (AR grade), KCl (AR grade), acetone (AR grade) and ethanol (SP grade) were obtained from Sinopharm Chemical Reagent Co., Ltd. All the chemicals were used without further purification. Au powders (99.99%) and Cr powders (99.99%) were
purchased from General Research Institute for Nonferrous Metals. The Si wafer with 300 nm thickness of SiO2 film was obtained from Beijing Zhongxing Bairui Technology Co., Ltd. 2.2 Preparation of the nano-Au film by electron beam evaporation (EBE) The nano-Au film was fabricated on the Si wafer by EBE in the high vacuum of 4×10-4 Pa (SPD-400/600, Shanghai Superconductor Technology Co., Ltd). A 10 kW gun was used to evaporate the samples. The films were deposited at room temperature (~25°C). In order to guarantee quality of the nano-Au film, a rigorous surface cleaning process of the Si wafer is necessary before evaporation, to remove the adsorbed impurities. The Si wafer was successively cleaned with acetone, ethanol and ultrapure water in an ultrasonic bath, and then dried at room temperature in a vacuum oven. In order to enhance the adhesion of nano-Au films on Si wafers, a Cr under-layer was firstly deposited on the Si wafer surface by EBE with a beam current of 60 mA for 70 s. Then, the nano-Au film was deposited on the Cr layer with a beam current of 70 mA. The thickness of nano-Au films was controlled by the deposition time, adjusting from 7 min to 20 min. 2.3 Preparation of electrochemically roughened nano-Au (ER-Au) film Electrochemical roughening of the nano-Au film was performed with two successive potential pulses in 0.1 M KCl solutions: an oxidation pulse (with different potentials) for 5 s and a reduction pulse (+0.5 V) for 10 s. The oxidation potential was set from 1.0 V to 1.4 V, and the cyclic number of potential pulses was set in the range
from 3 to 15 cycles (as shown in Fig.S1 of supporting information). After the electrochemical roughening, the ER-Au film was washed with ultrapure water to remove adsorbed electrolyte, and then dried at room temperature in a vacuum oven. 2.4 Characterization Surface morphologies and thickness of Au films were observed by a field emission scanning electron microscope (FE-SEM, HITACHI SU8220). The thickness of Au films was measured on a fresh cross section of Au-coated Si wafers (Au/Si). SERS spectra were recorded by Horiba HR evolution Raman spectrometer and excited with a He-Ne gas-state laser (633 nm, 0.94 mW). The acquisition time of Raman spectra was 5 s with 3 accumulations, and the spectral resolution reached 0.65 cm-1. Before the formal testing, the Raman spectrometer was calibrated by a Si wafer with a characteristic Raman peak shift at 520.7 cm-1. All the electrochemical measurements were performed in a conventional three-electrode system by the Autolab electrochemical analyzer (PGSTAT302N) equipped with Nova 1.11 software at room temperature (~25 °C). The Au/Si was served as the working electrode. A bright Pt plate (1.0 cm×1.5 cm) and a saturated calomel electrode (SCE) were selected as the counter electrode and reference electrode, respectively. The electrolyte solution was 0.1 M KCl. The reference electrode was led to the surface of the working electrode through a Luggin capillary. 3. Results and discussions 3.1 Morphology characterization of the Au films From the SEM images in Fig.1, a flat and compact Au film was coated on the Si
wafer, which was composed by numerous nanoscale particles. It was worth noting that no Cr-layer signals were observed from the cross section of Au/Si (in insets of Fig.1) due to the extremely thin Cr layer (estimated thickness ~5 nm). The deposition time showed an obvious effect on the size of Au particles and the thickness of the Au films. As the deposition time increased from 7 min to 20 min, the size of Au particles increased from 10 nm to 30 nm (in diameter) and the thickness of the Au films increased from 25 nm to 190 nm. 3.2 Preparation of ER-Au films and evaluation of SERS performance It is well known that the microstructure of substrate materials has a crucial effect on the SERS performance. In our paper, the microstructure of the ER-Au film was controlled by electrochemically roughening parameters (i.e. oxidation potentials, cyclic numbers) and thickness of the Au film. Moreover, the SERS performance of ER-Au films was evaluated using R6G as the model analyte. 3.2.1 Effect of oxidation potentials As is well-known, based on the basic principle of electrochemistry, the potential plays a decisive role in the oxidation/reduction of materials. Moreover, the redox potentials have an obvious effect on the roughness of materials in the electrochemical roughening process [36, 37]. In order to confirm the redox potentials of Au films used in the electrochemical roughening process, the cyclic voltammetry was firstly performed in a potential range of -0.2–1.5 V. From the cyclic voltammogram of the Au film in Fig.2a, it was clearly seen that the onset potential of Au anodic dissolution/oxidation was ~0.8 V, and the oxidation peak of the Au film appeared at
~1.2 V. As the potential swept negatively, the electrodeposition of Au from AuCl4occurred (AuCl4 +3e- →Au+4Cl- ) [38], and a cathodic reduction peak located at ~0.57 -
V. Therefore, the effect of oxidation potentials on electrochemical roughening of Au films was performed in a range of 1.0–1.4 V. The aggregation of Au nanoparticles occurred and a rough surface was obtained for the ER-Au films during the electrochemical roughening process (Fig.2b). The characteristic peaks of R6G were clearly identified from the SERS spectra on the ER-Au films (Fig.2c). The Raman bands at 612, 774, 1185, 1311, 1362, 1507 and 1649 cm-1 were attributed to the C-C-C ring in-plane vibration mode, the C-H out of plane bending mode, the C-H in-plane bending mode, the N-H in-plane bending mode and the in-plane C-C stretching mode of the R6G molecule, respectively [35]. As compared with the smooth Au film without roughening, the ER-Au film showed an obvious enhancement of Raman signals (in Fig.2c). Moreover, the oxidation potentials of ER-Au films had a strong effect on SERS signals of 10-6 M R6G. The strongest SERS signal was observed at the oxidation potential of 1.2 V. The potential-dependence of SERS spectra has also been reported on noble and transition metal nanomaterials [39]. It is believed that the sizes of nanoparticles and nanogaps between the nanoparticles play a vital role in SRES performance [40, 41] due to the EM enhancement effect. In the electrochemical roughening process, the size of nanoparticles and nanogaps on the ER-Au film surface can be conveniently adjusted by the oxidation potentials. From SEM images of the ER-Au films (in Fig. 2b and Fig.S2 of supporting information), with the increase of oxidation potential from 1.0 V
to 1.2 V, the average size of nanoparticles gradually increased from 25 nm to 50 nm (in diameter). As the oxidation potential further increased from 1.2 V to 1.4 V, the nanogaps between nanoparticles obviously enlarged due to the formation of isolated islands from nanoparticles agglomeration. 3.2.2 Effect of cyclic numbers The cyclic number of the oxidation-reduction process has an important effect on the SERS performance [42]. The effect of cyclic numbers on the SERS signals of R6G was shown in Fig.3a with a fixed oxidation potential of 1.2 V and the Au film thickness of 130 nm. As the cyclic number of the oxidation-reduction process increased from 3 cycles to 15 cycles, the strongest SERS signal was observed on the ER-Au film with 8 cycles. Besides, the agglomeration of Au nanoparticles occurred during the continuous potential scanning, leading to an increase of the size of nanoparticles and nanogaps (Fig.3b-f). The isolated-island structure was also clearly observed as the cyclic number further increased from 10 cycles to 15 cycles (Fig.3d-f). 3.2.3 Effect of thickness of Au films The thickness of Au films is another important factor of the microstructure control in the electrochemical roughening process. The Au films with different thickness were roughened under a fixed oxidation potential of 1.2 V and a cyclic number of 8 cycles (Fig.4). Owing to the agglomeration of Au particles during the oxidation-reduction process, numerous scattered island structures were observed on the thinner Au films (e.g. thickness≤110 nm, in Fig.4a-c). As the thickness of Au films
increased from 130 nm to 190 nm, a continuous and rough structure was obtained (Fig.4d-f), leading to a stronger SERS signals of R6G (Fig. 4g). However, SERS signals of R6G decreased on the incomplete roughening surface of Au films with thickness of 160 nm and 190 nm. The strongest SERS signals of R6G were observed on the ER-Au film with thickness of 130 nm due to the smaller size of nanoparticles and suitable gaps between nanoparticles. 3.2.4 Local EM Field Simulation In order to elucidate the effect of the microstructure of ER-Au films prepared under different roughening parameters on the spatial distribution of the EM field, the finite element method in the COMSOL Multiphysics software was used to simulate with an excitation wavelength of 633nm. Based on the experimental results, a model of SiO2 (300 nm thickness)-Cr (5 nm thickness)-Au (25-200 nm thickness, step size of 25 nm) layered periodic array was established. The effect of Au nanoparticle radii (10-32 nm) and gap distance between nanoparticles (1-10 nm) on the EM field was shown in Fig. 5a, as the Au film thickness was fixed at 125 nm. An obvious size effect on EM field enhancement (E/E0) was observed, and more enhancement was obtained on the nanogaps between nanoparticles due to generating more “hot spots” [43]. From the simulation results of Au films with different thicknesses (in Fig.S3 of supporting information), the strongest local EM enhancement were mainly observed at a radius of 25-28 nm and gap of 2-6 nm. The maximum E/E0 value appeared at a radius of 26 nm and a gap of 4 nm under the Au film thickness of 125 nm, up to 2.90×102. According to the SEM results of ER-Au films under different roughening parameters,
the optimal size of nanoparticles and nanogap (a radius of 26 nm and a gap of 5 nm) was obtained at an oxidation potential of 1.2 V and cyclic number of 8 cycles with the Au film thickness of 130 nm, which was consistent with the experiment results of the SERS test on ER-Au films. The optimized structure of ER-Au films generated more localized and high-density hot spots throughout the substrate, especially in the gap between nanoparticles (see Fig.5b) [44]. Moreover, the enhancement factor (EF) was calculated to be 7.07×109 from (E/E0)4 in Fig.5a, which showed a good conformity with the following experimental value. Besides, the SERS performance shows a strong dependence on the excitation wavelength, as a strong LSPR is obtained with the Raman excitation wavelength adjacent to absorption peak of ER-Au substrate [45, 46]. From Fig.S4 of supporting information, the ER-Au film produced a stronger resonance at 633 nm compared with other excitation wavelength (450-900 nm). These results demonstrated that the strongest LSPR was obtained at 633 nm on the ER-Au substrate. 3.3 Detection performance of SERS The SERS detection of R6G was performed on the surface of ER-Au films in the concentrations range of 10-9-10-5 M. The optimized ER-Au film was prepared at an oxidation potential of 1.2 V with the cyclic number of 8 cycles and the Au film thickness of 130 nm. The SERS signals of R6G increased with the concentrations, as shown in Fig.6a. For quantitative analysis of R6G concentrations, the standard curve was obtained by the logarithm of SERS intensities against logarithm of R6G concentrations. A linear relationship was obtained in the concentration range of
10-9-10-5 M by the plot of LogI versus LogC in Fig.6b: LogI = 0.48LogC + 6.76 Where, LogI represents for the logarithm of the SERS intensity of R6G at 774 cm−1 and LogC is the logarithm of R6G concentrations. The detection limit is 7.08×10-11 M with a correlation coefficient of R2=0.99. The detection limit was calculated as three times the standard deviation of the blanks divided by the slope of the calibration graph [47]. As a typical qualitative evaluation method of the Raman signal enhancement, the EF for R6G on the substrate was calculated by the following equation [35]: EF = ( ISERS ⁄IOR ) × ( COR ⁄CSERS )
(1)
Where, ISERS and IOR correspond to the Raman intensities of R6G on the ER-Au film and Si wafer, respectively. CSERS and COR refer to the molar concentration of R6G on the ER-Au film (10-9 M) and Si wafer (10-1 M), respectively. In order to conveniently compare with previous works, the same Raman peak (774 cm-1) was used to estimate the EF [18, 48]. The ISERS and IOR values were obtained directly from the experimental Raman spectra. The EF of the optimized ER-Au film was calculated to be 2.45×108. In addition, the EFs of the other characteristic peaks of R6G were calculated according to the same equation (Eq.1) and were listed in Table S1 of supporting information. It was worth noting that all EFs of the characteristic peaks of R6G were higher than 108, which indicates that the ER-Au film is an excellent SERS material. In addition, in order to verify the potential application of ER-Au films in the trace detection of toxic substances, PNP was selected as a model analyte due to its
high toxicity and widespread application in industries. The detection of PNP with different concentrations (10-9-10-3 M) was performed in the presence of 10-6 M R6G. The Raman spectra in Fig.6c clearly indicated that the presence of PNP obviously affected the peak intensity of R6G. The intensities of R6G characteristic peaks decreased with the increase of PNP concentrations. A linear relationship was obtained in the concentration range of 10-9-10-3 M PNP by the plot of LogI versus LogCPNP in Fig.6d: LogI = -0.09LogCPNP + 3.05 Where, LogI represents for the logarithm of the SERS intensity of R6G at 774 cm−1 and LogCPNP is the logarithm of PNP concentrations. Besides, almost no visible intensity change of R6G characteristic peaks was induced with the PNP concentration lower than 10-9 M, so the detection limit of PNP was estimated to be 10-9 M. As compared with the previous reports (in Table 1) [18, 34, 35, 47, 49-52], the ER-Au film showed a relatively high EF and a good detection performance (e.g. a larger linear detection range and lower detection limit). However, the sensitivity still need to improve and the direct detection of phenolic compounds is required, which are the main purposes of our future work on the preparation of SERS composite materials. The reproducibility and uniformity of the substrate are important factors for the SERS quantitative detection. The reproducibility of the ER-Au films was investigated by the SERS spectra of 10−6 M R6G in the absence and presence of 10-7 M PNP from 12 random acquisition sites on 6 batches of the ER-Au substrates prepared under the same conditions (in Fig.7a and b). Raman shift and intensity of R6G characteristic
peaks showed no obvious changes from 12 measurements (in Fig.S5 of supporting information).The relative standard deviation (RSD) values of the measured intensities of R6G characteristic peaks were all below 6.88% (insets of Fig.7a and b). These results demonstrated the good reproducibility of the ER-Au substrate and the reliability of the electrochemical roughening technology. Besides, in order to validate the uniformity of the ER-Au film, the SERS spectra of 10−6 M R6G in the absence and presence of 10-7 M PNP were collected from 12 random sites on the same substrate, as shown in Fig. 7c and d, respectively. Similarly, no obvious changes of R6G characteristic peaks were observed from 12 measurements (in Fig.S6 of supporting information).The RSD values of measured intensities of R6G characteristic peaks were all below 11% (insets of Fig.7c and d), which indicated the high uniformity of the ER-Au substrates [53]. Stability is another important evaluation parameter of a high-performance SERS substrate in practical application. The long-term storage stability of the ER-Au substrate was investigated by the SERS detection of 10-6 M R6G in the absence and presence of 10-7 M PNP during 60 days (Fig.8). The ER-Au substrate was stored in a drying oven at room temperature when not being in use. The oxidation and denaturation of the substrate surface are considered to be the main reasons for the detection performance deterioration. Obviously, a decay of the SERS intensity was observed in the first 30 days (26.5% decrement for R6G and 23.3% decrement for PNP), and then a relatively stable signal was obtained after 30 days. The results indicated that the ER-Au substrate had a good stability for the SERS detection.
3.4 Analytical application In order to evaluate the practical application of the developed method, a recovery test was performed by analyzing the tap water samples using the standard addition method. Three different concentrations of R6G were spiked in the tap water samples and the recovery of were calculated according to the linear equation in Fig.6b. As shown in Table 2, the recovery of R6G ranged from 96% to 98%, demonstrating that this method is effective and applicable. The RSD values are below 9.0%, indicating a good reproducibility of the method. 4. Conclusions A high-performance SERS substrate was successfully prepared by the electrochemical roughening technology. The SERS performance of the ER-Au film was obviously affected by the electrochemical roughening parameters and thickness of the Au films. A strongest enhancement of Raman signals was obtained on the ER-Au film with a thickness of 130 nm under the electrochemical roughening at an oxidation potential of 1.2 V and cyclic number of 8 cycles. The EF of R6G on the ER-Au film reached 108. Besides, the ER-Au film showed an ultrasensitive detection of R6G and PNP with a broad linear range of 10-9-10-5 M and 10-9-10-3 M, respectively. Furthermore, a good reproducibility and stability were obtained on the ER-Au film. Conflict of interest The authors declared that they have no conflicts of interest to this work. Acknowledgements This work is supported by the National Natural Science Foundation of China (no.
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Figure captions Fig. 1. SEM images of Au films with different deposition time: (a) 7 min, (b) 8 min, (c) 9 min, (d) 12 min, (e) 15 min and (f) 20 min. Inset of each figure is the thickness of Au films. Fig. 2. (a) Cyclic voltammograms of the Au film and bulk-Au in 0.1 M KCl solutions at a scan rate of 100 mV s-1. (b) SEM images of the ER-Au film with the thickness of 130 nm at oxidation potential of 1.2 V and cyclic number of 8 cycles. (c) Potential-dependent Raman spectra of 10-6 M R6G on the ER-Au film. Fig. 3. (a) Effect of cyclic numbers on Raman signals of 10-6 M R6G on the ER-Au film. (b-c) SEM images of the ER-Au films with different cyclic numbers at a fixed oxidation potential of 1.2 V and the thickness of 130 nm: (b) 3 cycles, (c) 5 cycles, (d) 10 cycles, (e) 12 cycles and (f) 15 cycles. Fig. 4. (a-e) SEM images of the ER-Au films with different thickness at an oxidation of 1.2 V and cyclic number of 8 cycles: (a) 25 nm, (b) 51 nm, (c) 110 nm, (d) 130 nm, (e) 160 nm and (f) 190 nm. (g) Effect of Au film thickness on Raman signals of 10-6 M R6G on the ER-Au films. Fig. 5. A simulated EM field distribution of the ER-Au film in a periodic SiO2-Cr-Au layered array nanostructure at 633 nm with the Au film thickness of 125 nm: (a) with different radius of Au nanoparticles (10-32 nm) and gap distance between nanoparticles (1-10 nm). (b) with a fixed radius of Au nanoparticles (26 nm) and gap distance between nanoparticles (4 nm). Fig. 6. (a) SERS spectra of R6G with varying concentrations of 10-9-10-5 M on the
optimized ER-Au film. (b) Corresponding calibration plots for quantitative analysis of R6G at 774 cm-1. (c) SERS spectra of 10−6 M R6G with different PNP concentrations of 10-9-10-3 M on the optimized ER-Au film. (d) Corresponding calibration plots for quantitative analysis of PNP at 774 cm-1. Fig. 7. (a,b) SERS spectra of 10−6 M R6G on 12 random sites from six different optimized ER-Au films: (a) in the absence of 10-7 M PNP and (b) in the presence of 10-7 M PNP. (c,d) SERS spectra of 10−6 M R6G on 12 random sites from the same ER-Au film: (c) in the absence of 10-7 M PNP and (d) the presence of 10-7 M PNP. Insets are the RSD values of R6G characteristic peak intensities. Fig. 8. The intensity change of Raman peak at 612 cm-1 of 10−6 M R6G in the absence and presence of 10-7 M PNP over 60 days: (a) in the absence of PNP and (b) in the presence of PNP.
Table1 Comparison of detection performance of different SERS substrates SERS
Preparation
linear
Detection
range (M)
limit (M)
R6G
10-3-10-7
10-7
2.8×106
[18]
Adenine
10-7-10-3
10-7
4.8×106
[34]
R6G
10-5-10-9
10-9
7.3×108
[35]
Analyte substrate
method Magnetron
Au/DW
EF Ref.
sputtering GO/Au
CVD fs laser
Cu-Ag processing Au/
Chemical
CV
10-5-10-9
3.8×10-11
3.4×107
PAOCG
reduction
4-Mpy
10-4-10-8
2.2×10-10
4.6×106
DNAs
10-11-10-8
10-11
2.6×103
[49]
ClO4−
10-5-10-6
10-6
__
[50]
HCHO
10-8-10-6
7.2×10-9
3.6×104
[51]
10-9
__
[52]
Thermal Au NW
[47]
evaporation Chemical Au–SiO2 reduction Au dendrites
Electro-depositi on
Au NPs/ Hydrothermal
Pb
2+
rGO
5×10-94×10
-6
Electrochemical
R6G
10-9-10-5
7.1×10-11
roughening
PNP
10-9-10-3
10-9
ER-Au
2.5×108
This work
Table 2 Detection of R6G spiked into tap water. Concentration(M) Sample
Recovery (%)
R6G
a
RSD (%)
a
Added
Detected
5×10-6
4.86×10-6
97.2
7.3
1×10-6
0.98×10-6
98.0
8.2
5×10-7
4.80×10-7
96.0
9.0
The mean value of five measurements.
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Highlights: 1. A potential-pulse technology was used to roughen the Au films. 2. A strong SERS effect with an enhancement factor of 108 was achieved on the roughened Au films. 3. SERS signals were affected by the electrochemical parameters and thickness of Au films. 4. A sensitive detection of R6G was achieved in a linear range of 10-9-10-5 M with detection limit of 10-11 M. 5. Quantitative analysis of PNP was realized in a wide linear range of 10-9-10-3 M.
Declaration of Interest Statement The authors declare no competing financial interest.
S0039-9140(19)31264-0
DOI:
https://doi.org/10.1016/j.talanta.2019.120631
Reference:
TAL 120631
To appear in:
Talanta
Received Date: 11 September 2019 Revised Date:
30 November 2019
Accepted Date: 7 December 2019
Please cite this article as: J. Wang, C. Qiu, X. Mu, H. Pang, X. Chen, D. Liu, Ultrasensitive SERS detection of rhodamine 6G and p-nitrophenol based on electrochemically roughened nano-Au film, Talanta (2020), doi: https://doi.org/10.1016/j.talanta.2019.120631. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.
Ultrasensitive SERS detection of rhodamine 6G and p-nitrophenol based on electrochemically roughened nano-Au film
Jiangcai Wanga, Cuicui Qiu*a,b, Xijiao Mu,c Hua Panga, Xinchun Chena, Dameng Liu∗a,b
a
State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
b
Tianjin Research Institute for Advanced Equipment, Tsinghua University, Tianjin
300300, China c
Beijing Key Laboratory for Magneto-Photoelectrical Composite and Interface
Science, School of Mathematics and Physics, University of Science and Technology Beijing, Beijing 100083, China ∗
Corresponding author:
Cuicui Qiu, email: [email protected] Dameng Liu, Tel: 86-10-62797646; email: [email protected]
Abstract Quantitative analysis of organic pollutants in environmental water is an important issue for ecological environment and human health. In this paper, the quantitative analysis of rhodamine 6G (R6G) and p-nitrophenol (PNP) is performed by the surface enhanced Raman scattering (SERS) technology. The enhancement of Raman signals is achieved on the surface of an electrochemically roughened nano-Au film. The SERS performance depends on the microstructure of roughened nano-Au films, which is affected by the thickness of Au films and electrochemical roughening parameters. The structure-dependence of SERS performance is validated by finite element simulation of local electromagnetic field distribution. An obvious SERS effect of R6G with an enhancement factor of 108 is obtained on the roughened nano-Au film. A sensitive SERS detection of R6G with a linear range of 10-9-10-5 M and a detection limit of 10-11 M is realized. Moreover, a wide linear range of 10-9-10-3 M is obtained for the detection of PNP. The roughened nano-Au film is an effective substrate for the SERS detection of organic pollutants with high reproducibility and good stability. Therefore, the electrochemical technology in this study is expected to be a very promising method for the fabrication of high-performance SERS substrate.
Keywords: SERS detection; organic pollutants; electrochemical roughening; nano-Au
1. Introduction In recent years, as the water pollution is getting more serious, the number and type of pollutants in environmental water are increasing. The organics (like dye, phenols, pesticides, etc.) with high toxicity are common pollutants in the water environment, which cause a severe threat to human health and ecological environment [1-3]. The organic pollutants usually come from manufacturing industry, such as pesticides, pharmaceuticals, synthetic dyes, explosives, etc. Once the organic pollutants are exposed in the environment, the potential risks of pathopoiesis and carcinogenesis cannot be ignored [4]. The detection of organic pollutants has always been an important issue in the field of environmental protection. The urgent demand for the detection of organic pollutants has stimulated development of analytical techniques. Various detection techniques have been reported in the past decades, including spectroscopic techniques (like fluorescence spectroscopy [5], infrared spectroscopy [6], Raman spectroscopy [7] and UV-vis Spectroscopy [8]), electroanalytical techniques [9], mass-spectrometric techniques [10], etc. As a powerful detection method, Raman spectroscopy has been widely applied in the fields of material science, chemistry, biological and biomedical science. However, practical applications of Raman spectroscopy are largely limited due to its low signal intensity. Therefore, the enhancement of Raman signals is vital for the sensitive detection. Surface enhanced Raman scattering (SERS) technology with a strong Raman signal has been widely applied in ultra-sensitive chemical and biological analysis [11,
12]. The Raman signals of adsorbed molecules on the substrate surface can be significantly enhanced approximately 108-1011 due to the electromagnetic (EM) enhancement effect [13]. The EM enhancement effect plays a decisive role in the SERS phenomenon, which benefits from localized surface plasmons resonance (LSPR) of the metal nanoparticles [14, 15]. The EM enhancement mainly depends on the type and microstructure of substrate materials. The SERS substrate materials have been focused on Ag [16, 17], Au [18, 19], Cu [20, 21] and Ni [22], which own the suitable dielectric constant and strong surface plasmon band. It is well known that nanostructured Ag shows the strongest SERS effect. However, Ag surface is susceptible to oxidation/denaturation in the air/solutions. Thus, the nanostructured Au can become a promising alternative material due to its superior stability and strong plasmon band [23, 24]. The nanostructured Au has widely used in the fields of environment, biology, medicine and etc. It has been reported that the nanostructured Au with a large surface area can effectively improve the sensitivity of the SERS signals due to the increase of hot spots [25, 26]. Recently, a lot of works have focused on the SERS detection of organic pollutant in environment based on the nanostructured Au, like dye [27], pesticide residues [28], bisphenol A [29], persistent organic pollutant [30], cannabinol [31] and etc. As a major class of organic pollutants, phenolic compounds with high toxicity and persistence have attracted great attention in chemical and environmental fields. However, the direct SERS detection of phenolic compounds was limited on the metal nanomaterials surface due to a very weak Raman signal at low concentrations [32]. A
variety of nanostructured Au are designed to enhance the SERS performance by different preparation techniques (like hydrothermal method [33], chemical vapor deposition [34], and all femtosecond laser processing technique [35], etc.). However, some preparation techniques is too complicated and time-consuming for the practical application. Therefore, a facile and low-cost method is necessary for the preparation of high-performance SERS substrate materials. Considering that electrochemical technology is a facile method to fabricate nanomaterials, in this paper, a potential-step technology was used to prepare the high-performance nano-Au SERS substrate. During the electrochemical roughening process, the effect of the thickness of Au film and electrochemical parameters (like oxidation potentials and cyclic numbers) on the microstructure of roughened nano-Au films was investigated. Moreover, the microstructure of roughened nano-Au films had a significant effect on the SERS performance of R6G analyte molecules, which was validated by finite element simulation. Furthermore, SERS detection performance of R6G and p-nitrophenol (PNP) on the electrochemically roughened nano-Au was evaluated. 2. Experimental 2.1 Materials and regents R6G (AR grade) was purchased from Shanghai Macklin Biochemical Co., Ltd. PNP (AR grade), KCl (AR grade), acetone (AR grade) and ethanol (SP grade) were obtained from Sinopharm Chemical Reagent Co., Ltd. All the chemicals were used without further purification. Au powders (99.99%) and Cr powders (99.99%) were
purchased from General Research Institute for Nonferrous Metals. The Si wafer with 300 nm thickness of SiO2 film was obtained from Beijing Zhongxing Bairui Technology Co., Ltd. 2.2 Preparation of the nano-Au film by electron beam evaporation (EBE) The nano-Au film was fabricated on the Si wafer by EBE in the high vacuum of 4×10-4 Pa (SPD-400/600, Shanghai Superconductor Technology Co., Ltd). A 10 kW gun was used to evaporate the samples. The films were deposited at room temperature (~25°C). In order to guarantee quality of the nano-Au film, a rigorous surface cleaning process of the Si wafer is necessary before evaporation, to remove the adsorbed impurities. The Si wafer was successively cleaned with acetone, ethanol and ultrapure water in an ultrasonic bath, and then dried at room temperature in a vacuum oven. In order to enhance the adhesion of nano-Au films on Si wafers, a Cr under-layer was firstly deposited on the Si wafer surface by EBE with a beam current of 60 mA for 70 s. Then, the nano-Au film was deposited on the Cr layer with a beam current of 70 mA. The thickness of nano-Au films was controlled by the deposition time, adjusting from 7 min to 20 min. 2.3 Preparation of electrochemically roughened nano-Au (ER-Au) film Electrochemical roughening of the nano-Au film was performed with two successive potential pulses in 0.1 M KCl solutions: an oxidation pulse (with different potentials) for 5 s and a reduction pulse (+0.5 V) for 10 s. The oxidation potential was set from 1.0 V to 1.4 V, and the cyclic number of potential pulses was set in the range
from 3 to 15 cycles (as shown in Fig.S1 of supporting information). After the electrochemical roughening, the ER-Au film was washed with ultrapure water to remove adsorbed electrolyte, and then dried at room temperature in a vacuum oven. 2.4 Characterization Surface morphologies and thickness of Au films were observed by a field emission scanning electron microscope (FE-SEM, HITACHI SU8220). The thickness of Au films was measured on a fresh cross section of Au-coated Si wafers (Au/Si). SERS spectra were recorded by Horiba HR evolution Raman spectrometer and excited with a He-Ne gas-state laser (633 nm, 0.94 mW). The acquisition time of Raman spectra was 5 s with 3 accumulations, and the spectral resolution reached 0.65 cm-1. Before the formal testing, the Raman spectrometer was calibrated by a Si wafer with a characteristic Raman peak shift at 520.7 cm-1. All the electrochemical measurements were performed in a conventional three-electrode system by the Autolab electrochemical analyzer (PGSTAT302N) equipped with Nova 1.11 software at room temperature (~25 °C). The Au/Si was served as the working electrode. A bright Pt plate (1.0 cm×1.5 cm) and a saturated calomel electrode (SCE) were selected as the counter electrode and reference electrode, respectively. The electrolyte solution was 0.1 M KCl. The reference electrode was led to the surface of the working electrode through a Luggin capillary. 3. Results and discussions 3.1 Morphology characterization of the Au films From the SEM images in Fig.1, a flat and compact Au film was coated on the Si
wafer, which was composed by numerous nanoscale particles. It was worth noting that no Cr-layer signals were observed from the cross section of Au/Si (in insets of Fig.1) due to the extremely thin Cr layer (estimated thickness ~5 nm). The deposition time showed an obvious effect on the size of Au particles and the thickness of the Au films. As the deposition time increased from 7 min to 20 min, the size of Au particles increased from 10 nm to 30 nm (in diameter) and the thickness of the Au films increased from 25 nm to 190 nm. 3.2 Preparation of ER-Au films and evaluation of SERS performance It is well known that the microstructure of substrate materials has a crucial effect on the SERS performance. In our paper, the microstructure of the ER-Au film was controlled by electrochemically roughening parameters (i.e. oxidation potentials, cyclic numbers) and thickness of the Au film. Moreover, the SERS performance of ER-Au films was evaluated using R6G as the model analyte. 3.2.1 Effect of oxidation potentials As is well-known, based on the basic principle of electrochemistry, the potential plays a decisive role in the oxidation/reduction of materials. Moreover, the redox potentials have an obvious effect on the roughness of materials in the electrochemical roughening process [36, 37]. In order to confirm the redox potentials of Au films used in the electrochemical roughening process, the cyclic voltammetry was firstly performed in a potential range of -0.2–1.5 V. From the cyclic voltammogram of the Au film in Fig.2a, it was clearly seen that the onset potential of Au anodic dissolution/oxidation was ~0.8 V, and the oxidation peak of the Au film appeared at
~1.2 V. As the potential swept negatively, the electrodeposition of Au from AuCl4occurred (AuCl4 +3e- →Au+4Cl- ) [38], and a cathodic reduction peak located at ~0.57 -
V. Therefore, the effect of oxidation potentials on electrochemical roughening of Au films was performed in a range of 1.0–1.4 V. The aggregation of Au nanoparticles occurred and a rough surface was obtained for the ER-Au films during the electrochemical roughening process (Fig.2b). The characteristic peaks of R6G were clearly identified from the SERS spectra on the ER-Au films (Fig.2c). The Raman bands at 612, 774, 1185, 1311, 1362, 1507 and 1649 cm-1 were attributed to the C-C-C ring in-plane vibration mode, the C-H out of plane bending mode, the C-H in-plane bending mode, the N-H in-plane bending mode and the in-plane C-C stretching mode of the R6G molecule, respectively [35]. As compared with the smooth Au film without roughening, the ER-Au film showed an obvious enhancement of Raman signals (in Fig.2c). Moreover, the oxidation potentials of ER-Au films had a strong effect on SERS signals of 10-6 M R6G. The strongest SERS signal was observed at the oxidation potential of 1.2 V. The potential-dependence of SERS spectra has also been reported on noble and transition metal nanomaterials [39]. It is believed that the sizes of nanoparticles and nanogaps between the nanoparticles play a vital role in SRES performance [40, 41] due to the EM enhancement effect. In the electrochemical roughening process, the size of nanoparticles and nanogaps on the ER-Au film surface can be conveniently adjusted by the oxidation potentials. From SEM images of the ER-Au films (in Fig. 2b and Fig.S2 of supporting information), with the increase of oxidation potential from 1.0 V
to 1.2 V, the average size of nanoparticles gradually increased from 25 nm to 50 nm (in diameter). As the oxidation potential further increased from 1.2 V to 1.4 V, the nanogaps between nanoparticles obviously enlarged due to the formation of isolated islands from nanoparticles agglomeration. 3.2.2 Effect of cyclic numbers The cyclic number of the oxidation-reduction process has an important effect on the SERS performance [42]. The effect of cyclic numbers on the SERS signals of R6G was shown in Fig.3a with a fixed oxidation potential of 1.2 V and the Au film thickness of 130 nm. As the cyclic number of the oxidation-reduction process increased from 3 cycles to 15 cycles, the strongest SERS signal was observed on the ER-Au film with 8 cycles. Besides, the agglomeration of Au nanoparticles occurred during the continuous potential scanning, leading to an increase of the size of nanoparticles and nanogaps (Fig.3b-f). The isolated-island structure was also clearly observed as the cyclic number further increased from 10 cycles to 15 cycles (Fig.3d-f). 3.2.3 Effect of thickness of Au films The thickness of Au films is another important factor of the microstructure control in the electrochemical roughening process. The Au films with different thickness were roughened under a fixed oxidation potential of 1.2 V and a cyclic number of 8 cycles (Fig.4). Owing to the agglomeration of Au particles during the oxidation-reduction process, numerous scattered island structures were observed on the thinner Au films (e.g. thickness≤110 nm, in Fig.4a-c). As the thickness of Au films
increased from 130 nm to 190 nm, a continuous and rough structure was obtained (Fig.4d-f), leading to a stronger SERS signals of R6G (Fig. 4g). However, SERS signals of R6G decreased on the incomplete roughening surface of Au films with thickness of 160 nm and 190 nm. The strongest SERS signals of R6G were observed on the ER-Au film with thickness of 130 nm due to the smaller size of nanoparticles and suitable gaps between nanoparticles. 3.2.4 Local EM Field Simulation In order to elucidate the effect of the microstructure of ER-Au films prepared under different roughening parameters on the spatial distribution of the EM field, the finite element method in the COMSOL Multiphysics software was used to simulate with an excitation wavelength of 633nm. Based on the experimental results, a model of SiO2 (300 nm thickness)-Cr (5 nm thickness)-Au (25-200 nm thickness, step size of 25 nm) layered periodic array was established. The effect of Au nanoparticle radii (10-32 nm) and gap distance between nanoparticles (1-10 nm) on the EM field was shown in Fig. 5a, as the Au film thickness was fixed at 125 nm. An obvious size effect on EM field enhancement (E/E0) was observed, and more enhancement was obtained on the nanogaps between nanoparticles due to generating more “hot spots” [43]. From the simulation results of Au films with different thicknesses (in Fig.S3 of supporting information), the strongest local EM enhancement were mainly observed at a radius of 25-28 nm and gap of 2-6 nm. The maximum E/E0 value appeared at a radius of 26 nm and a gap of 4 nm under the Au film thickness of 125 nm, up to 2.90×102. According to the SEM results of ER-Au films under different roughening parameters,
the optimal size of nanoparticles and nanogap (a radius of 26 nm and a gap of 5 nm) was obtained at an oxidation potential of 1.2 V and cyclic number of 8 cycles with the Au film thickness of 130 nm, which was consistent with the experiment results of the SERS test on ER-Au films. The optimized structure of ER-Au films generated more localized and high-density hot spots throughout the substrate, especially in the gap between nanoparticles (see Fig.5b) [44]. Moreover, the enhancement factor (EF) was calculated to be 7.07×109 from (E/E0)4 in Fig.5a, which showed a good conformity with the following experimental value. Besides, the SERS performance shows a strong dependence on the excitation wavelength, as a strong LSPR is obtained with the Raman excitation wavelength adjacent to absorption peak of ER-Au substrate [45, 46]. From Fig.S4 of supporting information, the ER-Au film produced a stronger resonance at 633 nm compared with other excitation wavelength (450-900 nm). These results demonstrated that the strongest LSPR was obtained at 633 nm on the ER-Au substrate. 3.3 Detection performance of SERS The SERS detection of R6G was performed on the surface of ER-Au films in the concentrations range of 10-9-10-5 M. The optimized ER-Au film was prepared at an oxidation potential of 1.2 V with the cyclic number of 8 cycles and the Au film thickness of 130 nm. The SERS signals of R6G increased with the concentrations, as shown in Fig.6a. For quantitative analysis of R6G concentrations, the standard curve was obtained by the logarithm of SERS intensities against logarithm of R6G concentrations. A linear relationship was obtained in the concentration range of
10-9-10-5 M by the plot of LogI versus LogC in Fig.6b: LogI = 0.48LogC + 6.76 Where, LogI represents for the logarithm of the SERS intensity of R6G at 774 cm−1 and LogC is the logarithm of R6G concentrations. The detection limit is 7.08×10-11 M with a correlation coefficient of R2=0.99. The detection limit was calculated as three times the standard deviation of the blanks divided by the slope of the calibration graph [47]. As a typical qualitative evaluation method of the Raman signal enhancement, the EF for R6G on the substrate was calculated by the following equation [35]: EF = ( ISERS ⁄IOR ) × ( COR ⁄CSERS )
(1)
Where, ISERS and IOR correspond to the Raman intensities of R6G on the ER-Au film and Si wafer, respectively. CSERS and COR refer to the molar concentration of R6G on the ER-Au film (10-9 M) and Si wafer (10-1 M), respectively. In order to conveniently compare with previous works, the same Raman peak (774 cm-1) was used to estimate the EF [18, 48]. The ISERS and IOR values were obtained directly from the experimental Raman spectra. The EF of the optimized ER-Au film was calculated to be 2.45×108. In addition, the EFs of the other characteristic peaks of R6G were calculated according to the same equation (Eq.1) and were listed in Table S1 of supporting information. It was worth noting that all EFs of the characteristic peaks of R6G were higher than 108, which indicates that the ER-Au film is an excellent SERS material. In addition, in order to verify the potential application of ER-Au films in the trace detection of toxic substances, PNP was selected as a model analyte due to its
high toxicity and widespread application in industries. The detection of PNP with different concentrations (10-9-10-3 M) was performed in the presence of 10-6 M R6G. The Raman spectra in Fig.6c clearly indicated that the presence of PNP obviously affected the peak intensity of R6G. The intensities of R6G characteristic peaks decreased with the increase of PNP concentrations. A linear relationship was obtained in the concentration range of 10-9-10-3 M PNP by the plot of LogI versus LogCPNP in Fig.6d: LogI = -0.09LogCPNP + 3.05 Where, LogI represents for the logarithm of the SERS intensity of R6G at 774 cm−1 and LogCPNP is the logarithm of PNP concentrations. Besides, almost no visible intensity change of R6G characteristic peaks was induced with the PNP concentration lower than 10-9 M, so the detection limit of PNP was estimated to be 10-9 M. As compared with the previous reports (in Table 1) [18, 34, 35, 47, 49-52], the ER-Au film showed a relatively high EF and a good detection performance (e.g. a larger linear detection range and lower detection limit). However, the sensitivity still need to improve and the direct detection of phenolic compounds is required, which are the main purposes of our future work on the preparation of SERS composite materials. The reproducibility and uniformity of the substrate are important factors for the SERS quantitative detection. The reproducibility of the ER-Au films was investigated by the SERS spectra of 10−6 M R6G in the absence and presence of 10-7 M PNP from 12 random acquisition sites on 6 batches of the ER-Au substrates prepared under the same conditions (in Fig.7a and b). Raman shift and intensity of R6G characteristic
peaks showed no obvious changes from 12 measurements (in Fig.S5 of supporting information).The relative standard deviation (RSD) values of the measured intensities of R6G characteristic peaks were all below 6.88% (insets of Fig.7a and b). These results demonstrated the good reproducibility of the ER-Au substrate and the reliability of the electrochemical roughening technology. Besides, in order to validate the uniformity of the ER-Au film, the SERS spectra of 10−6 M R6G in the absence and presence of 10-7 M PNP were collected from 12 random sites on the same substrate, as shown in Fig. 7c and d, respectively. Similarly, no obvious changes of R6G characteristic peaks were observed from 12 measurements (in Fig.S6 of supporting information).The RSD values of measured intensities of R6G characteristic peaks were all below 11% (insets of Fig.7c and d), which indicated the high uniformity of the ER-Au substrates [53]. Stability is another important evaluation parameter of a high-performance SERS substrate in practical application. The long-term storage stability of the ER-Au substrate was investigated by the SERS detection of 10-6 M R6G in the absence and presence of 10-7 M PNP during 60 days (Fig.8). The ER-Au substrate was stored in a drying oven at room temperature when not being in use. The oxidation and denaturation of the substrate surface are considered to be the main reasons for the detection performance deterioration. Obviously, a decay of the SERS intensity was observed in the first 30 days (26.5% decrement for R6G and 23.3% decrement for PNP), and then a relatively stable signal was obtained after 30 days. The results indicated that the ER-Au substrate had a good stability for the SERS detection.
3.4 Analytical application In order to evaluate the practical application of the developed method, a recovery test was performed by analyzing the tap water samples using the standard addition method. Three different concentrations of R6G were spiked in the tap water samples and the recovery of were calculated according to the linear equation in Fig.6b. As shown in Table 2, the recovery of R6G ranged from 96% to 98%, demonstrating that this method is effective and applicable. The RSD values are below 9.0%, indicating a good reproducibility of the method. 4. Conclusions A high-performance SERS substrate was successfully prepared by the electrochemical roughening technology. The SERS performance of the ER-Au film was obviously affected by the electrochemical roughening parameters and thickness of the Au films. A strongest enhancement of Raman signals was obtained on the ER-Au film with a thickness of 130 nm under the electrochemical roughening at an oxidation potential of 1.2 V and cyclic number of 8 cycles. The EF of R6G on the ER-Au film reached 108. Besides, the ER-Au film showed an ultrasensitive detection of R6G and PNP with a broad linear range of 10-9-10-5 M and 10-9-10-3 M, respectively. Furthermore, a good reproducibility and stability were obtained on the ER-Au film. Conflict of interest The authors declared that they have no conflicts of interest to this work. Acknowledgements This work is supported by the National Natural Science Foundation of China (no.
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Figure captions Fig. 1. SEM images of Au films with different deposition time: (a) 7 min, (b) 8 min, (c) 9 min, (d) 12 min, (e) 15 min and (f) 20 min. Inset of each figure is the thickness of Au films. Fig. 2. (a) Cyclic voltammograms of the Au film and bulk-Au in 0.1 M KCl solutions at a scan rate of 100 mV s-1. (b) SEM images of the ER-Au film with the thickness of 130 nm at oxidation potential of 1.2 V and cyclic number of 8 cycles. (c) Potential-dependent Raman spectra of 10-6 M R6G on the ER-Au film. Fig. 3. (a) Effect of cyclic numbers on Raman signals of 10-6 M R6G on the ER-Au film. (b-c) SEM images of the ER-Au films with different cyclic numbers at a fixed oxidation potential of 1.2 V and the thickness of 130 nm: (b) 3 cycles, (c) 5 cycles, (d) 10 cycles, (e) 12 cycles and (f) 15 cycles. Fig. 4. (a-e) SEM images of the ER-Au films with different thickness at an oxidation of 1.2 V and cyclic number of 8 cycles: (a) 25 nm, (b) 51 nm, (c) 110 nm, (d) 130 nm, (e) 160 nm and (f) 190 nm. (g) Effect of Au film thickness on Raman signals of 10-6 M R6G on the ER-Au films. Fig. 5. A simulated EM field distribution of the ER-Au film in a periodic SiO2-Cr-Au layered array nanostructure at 633 nm with the Au film thickness of 125 nm: (a) with different radius of Au nanoparticles (10-32 nm) and gap distance between nanoparticles (1-10 nm). (b) with a fixed radius of Au nanoparticles (26 nm) and gap distance between nanoparticles (4 nm). Fig. 6. (a) SERS spectra of R6G with varying concentrations of 10-9-10-5 M on the
optimized ER-Au film. (b) Corresponding calibration plots for quantitative analysis of R6G at 774 cm-1. (c) SERS spectra of 10−6 M R6G with different PNP concentrations of 10-9-10-3 M on the optimized ER-Au film. (d) Corresponding calibration plots for quantitative analysis of PNP at 774 cm-1. Fig. 7. (a,b) SERS spectra of 10−6 M R6G on 12 random sites from six different optimized ER-Au films: (a) in the absence of 10-7 M PNP and (b) in the presence of 10-7 M PNP. (c,d) SERS spectra of 10−6 M R6G on 12 random sites from the same ER-Au film: (c) in the absence of 10-7 M PNP and (d) the presence of 10-7 M PNP. Insets are the RSD values of R6G characteristic peak intensities. Fig. 8. The intensity change of Raman peak at 612 cm-1 of 10−6 M R6G in the absence and presence of 10-7 M PNP over 60 days: (a) in the absence of PNP and (b) in the presence of PNP.
Table1 Comparison of detection performance of different SERS substrates SERS
Preparation
linear
Detection
range (M)
limit (M)
R6G
10-3-10-7
10-7
2.8×106
[18]
Adenine
10-7-10-3
10-7
4.8×106
[34]
R6G
10-5-10-9
10-9
7.3×108
[35]
Analyte substrate
method Magnetron
Au/DW
EF Ref.
sputtering GO/Au
CVD fs laser
Cu-Ag processing Au/
Chemical
CV
10-5-10-9
3.8×10-11
3.4×107
PAOCG
reduction
4-Mpy
10-4-10-8
2.2×10-10
4.6×106
DNAs
10-11-10-8
10-11
2.6×103
[49]
ClO4−
10-5-10-6
10-6
__
[50]
HCHO
10-8-10-6
7.2×10-9
3.6×104
[51]
10-9
__
[52]
Thermal Au NW
[47]
evaporation Chemical Au–SiO2 reduction Au dendrites
Electro-depositi on
Au NPs/ Hydrothermal
Pb
2+
rGO
5×10-94×10
-6
Electrochemical
R6G
10-9-10-5
7.1×10-11
roughening
PNP
10-9-10-3
10-9
ER-Au
2.5×108
This work
Table 2 Detection of R6G spiked into tap water. Concentration(M) Sample
Recovery (%)
R6G
a
RSD (%)
a
Added
Detected
5×10-6
4.86×10-6
97.2
7.3
1×10-6
0.98×10-6
98.0
8.2
5×10-7
4.80×10-7
96.0
9.0
The mean value of five measurements.
Fig. 1
Fig. 2
Fig. 3
Fig. 4
Fig. 5
Fig. 6
Fig. 7
Fig. 8
Highlights: 1. A potential-pulse technology was used to roughen the Au films. 2. A strong SERS effect with an enhancement factor of 108 was achieved on the roughened Au films. 3. SERS signals were affected by the electrochemical parameters and thickness of Au films. 4. A sensitive detection of R6G was achieved in a linear range of 10-9-10-5 M with detection limit of 10-11 M. 5. Quantitative analysis of PNP was realized in a wide linear range of 10-9-10-3 M.
Declaration of Interest Statement The authors declare no competing financial interest.