Received 3rd December 2013Accepted 10th March 2014
DOI: 10.1039/c3ay42153h
www.rsc.org/methods
4130 | Anal. Methods, 2014, 6, 4130–41
Ultrasensitive and reproducible surface-enhancedRaman scattering detection via an optimizedadsorption process and filter-based substrate†
Xiao-long Wang,ab Rui-long Zong,*a Shi-kao Shi*b and Yongfa Zhua
Wih a view to improve the surface-enhanced Raman scattering (SERS) properties, including the sensitivity
and uniformity, a simple and practical pretreatment method via optimizing the adsorption process of
probe molecules and the morphology of SERS-active nanostructures was studied. Excellent SERS
performances were obtained using this method such as high sensitivity (5 � 10�14 M rhodamine 6G, 1 �10�8 M melamine), remarkable uniformity (�9% relative standard deviation (RSD)) and reproducibility
(�10% RSD). Uniform and repeatable silver nanoparticle arrays were fabricated by an optimized filter-
based method, which guaranteed the reliability of SERS detection. The SERS intensity was greatly
improved by adjusting the adsorption capacity of the noble metal nanostructures, which has a linear
relationship with the pH value of silver solution below 10. An accurate and reliable enhancement factor
(EF) of 3.28 � 108 was calculated through the quantification of the adsorbed probe molecules. It is worth
noting that the lifetime of the substrates prepared by this method could be prolonged to two months,
which was definitely longer than other ordinary silver SERS substrates. The pretreatment method not
only provided perfect SERS properties, but also made SERS a practical analysis tool because of its good
homogeneity and ultralow detection limit.
1. Introduction
Surface-enhanced Raman scattering (SERS), as a trace analysismethod, has been extended into a variety of areas includingchemical and biological sensing.1–5 However, it is still far frombeing useful for practical applications.6,7 The uniformity,reproducibility, sensitivity and quantication are basicrequirements to transform the advanced method into practicalapplications.8,9 According to the most widely used denition ofenhancement factors (EFs): EF ¼ (ISERS/Nads)/(Iref/Nsol), theintensity of SERS signal, ISERS ¼ EFNads(Iref/Nsol), is only affectedby two factors: the EF, which is associated with the character-istics of the substrate, and the number of adsorbed probemolecules, which is related to the adsorption capacity ofnanostructures. The optimized noble metal nanostructure andthe adsorption process are both of crucial importance to theSERS performances.
Up to now, most research has been focused on the prepa-ration of high sensitive SERS substrates. A variety of preparation
ytical Methods and Instrumentation,
ersity, Beijing 100084, China. E-mail:
2781686
, Hebei Normal University, Shijiazhuang
ua.edu.cn; Tel: +86-311-80787402
tion (ESI) available. See DOI:
37
methods have been developed to fabricate SERS-activesubstrates, such as the electrochemical roughening of elec-trodes,10,11 lithography,12,13 template techniques,14,15 sol–gelmethod,16,17 self-assembly,18,19 and in situ chemical growth.20,21
Among these methods, the bottom–up processes such as sol–geland self-assembly offered several advantages such as highsensitivity, low cost, ease of fabrication, and diverse substratematerials. Moreover, such substrates had abundant “hot spots”in a local scope which greatly enhanced the SERS sensitivitiesdue to the aggregation of nanoparticles. Nevertheless, the maindrawbacks of these methods were poor uniformity and repro-ducibility, which limited their practical applications. Recently,several groups have attempted to improve the uniformity andreproducibility of SERS signals using a self-assembly methodbased on ltering.22,23 However, they did not form denselypackaged nanoparticle arrays using the straightforwardadsorption or the lter-based method. Hence, the uniformityand sensitivity of SERS signals could be improved further byoptimizing their morphology.
The surface chemistry of the noble metal nanostructuresplays a critical role in SERS.24–26 It is essential to take fulladvantage of the adsorption between probe molecules andnoble metal nanoparticles in particular in trace detection. Theadsorption process in the SERS detection can be classied intotwo types. (1) Adsorption before the setup of the SERSsubstrate: the probe molecules are mixed with noble metalnanoparticles and adequately adsorbed for some time, and
then some of the solution is dripped onto the substrate (glass,silicon, and metal foil et al.) to form a self-assembled nano-structure acting as a SERS-active substrate.16,27 Although thenumber of probe molecules in the solution can be wellcalculated, all molecules cannot fully adsorb on the particlesand the residual molecules will cause an uneven distributionaer drying. (2) Adsorption aer the setup of the SERSsubstrate: the solution of the probe molecules is dripped ontothe SERS substrate, or the substrate is immersed into thesolution of the probe molecules for some time. And then thesolution is dried whether the residue molecules are washed ornot.19,28 This type of adsorption guaranteed a perfect distri-bution of probe molecules. However, it is difficult to determinethe actual number of probe molecules which affects theaccurate calculation of EFs. The inuence of adsorption inSERS had been studied in the early literature.29,30 Adsorption atthe surface of the nanoparticles will take place when theanalyte is in contact with them, whose kinetics and thermo-dynamics processes are affected by their interaction such asthe electrostatic attraction or chemically bonding. Forexample, Fang et al. reported that the orientation of thechemically adsorbed probes is mainly affected by the pH.31 Duand co-workers showed that the adsorption of either cationicor anionic species on Ag NPs can be promoted by controllingthe surface charge on the particles.32 Exploring SERS sensingusing colloidal solutions, Aroca and co-workers found that theSERS intensity is strongly related to the zeta potential and fordifferent analytes.33 However, up to now the adsorptionprocess did not get adequate emphasis in SERS research,which neglects its important role in the theoretical and prac-tical applications of SERS.
In this paper, we discussed the development of an optimizedlter-based method to prepare SERS-active silver nanoparticlearrays with perfect uniformity and high sensitivity. Two keyparameters are thought to be responsible for the good perfor-mances of the SERS substrate – uniform morphology andadequate adsorption. Here we focused on controlling theuniformity and repeatability of the SERS substrate through asimple and reliable lter-based method. Uniform and adequateadsorption of probe molecules was obtained by optimizing thethermodynamic process of adsorption and the zeta potential ofsilver solution.
2. Experimental2.1. Reagents and materials
Silver nitrate was purchased from the Sigma-Aldrich Chem-ical Co. (St. Louis, MO). Poly(vinylpyrrolidone) (PVP; averagemolecular weight of 30 000 g mol�1), glucose, ammoniumhydroxide, sodium hydroxide, rhodamine 6G, and melaminewere purchased from Sinopharm Chemical Reagent Co. Ltd.(Shanghai, China). 13 mm diameter microltrationmembranes (Nylon) with 0.22 mm pore size were obtainedfrom Jinteng Chemical Reagent Co. Ltd. (Tianjin, China).All these chemicals were analytical grade and used asreceived.
All of the glass containers used for the synthesis and storage ofsilver nanoparticles (Ag NPs) were immersed in 30% nitric acid(v/v) and rinsed with deionized water. Briey, 200 mg PVP and100 mg glucose were put into a 500 mL conical ask with 150mL deionized water and were dissolved by ultrasonication.Then 50 mL NaOH (1 M) and 200 mL Au seeds were added. Auseeds of 5 nm were prepared by the Lee–Meisel method.34 TheTollens' reagent was prepared as followed: 50 mg AgNO3 wasput into a 100mL beaker and dissolved with 10mLH2O, then 30mL NaOH (1 M) was added and brown precipitates appeared inthe solution. A volume ratio of 1 : 20 of the ammonia solutionwas added dropwise into the solution until the precipitationdisappeared completely, and then the solution was diluted to 50mL with deionized water. Finally, the Tollens' reagent wasdropped into the solution containing Au seeds at a rate of 0.5mL min�1 by a syringe pump, and the solution was vigorouslystirred during the process. The nal product was stored at 4 �C.
2.3. Pretreatment of SERS analysis
In brief, 5 mL silver solutions were centrifuged at 12 000 rpmsfor 10 min before the addition of probe molecules. Aerremoving the supernatant containing the free PVP, the depos-ited silver nanoparticles on the bottom of the tube were redis-persed in 900 mL deionized water using ultrasonicator. Tooptimize the adsorption properties according to different typesof probe molecules, the surface state of silver nanoparticles wasadjusted by acid or alkali to the desired pH value. Then, 100 mLprobe molecules were added and shaken using a vortex shakerfor adequate adsorption. The mixture was ltered with a 13 mmdiameter lter membrane. Removing the outer support of thelter membrane, the SERS-active arrays supported on the ltermembrane were mounted on a slide, which could be used forSERS analysis.
2.4. Characterization
Ag nanoparticles were imaged using a HITACHI HT7700transmission electron microscope (TEM) operated at an accel-erating voltage of 100 kV. Surface morphologies and structuresof the SERS-active arrays were examined with a LEO-1530 eldemission scanning electron microscope (FE-SEM) operated atan accelerating voltage of 10 kV. The size distribution and zetapotential of Ag NPs were characterized by a dynamic lightscattering method (DLS) using HORIBA SZ-100 analyzer. TheUV-vis absorption spectra of the Ag nanoparticles were recordedin the range from 300 to 800 nm using a HITACHI U-3010spectroscope. Steady-state uorescence spectra (FS) wererecorded with a HORIBA Aqualog Fluorescence Spectrometer. X-ray photo-electron spectroscopy (XPS) was measured in a PHIQuantera SXM system. The binding energies were normalized tothe signal for amorphous carbon at 284.8 eV. Raman spectrawere recorded using a HORIBA JY HR800 confocal microscopeRaman spectrometer employing an Ar–ion laser operating at514 nm with an output power of 0.01 mW on the sample. A 50�telephoto Olympus objective lens was used to focus the laser on
the samples (laser spot size: ca. 2 mm). All spectra were cali-brated with respect to a silicon wafer at 520.7 cm�1. Theacquisition time for each measurement was 2 s and the scans toco-add was 2 unless otherwise specied.
3. Results and discussion3.1. Preparation and characterization of Ag NPs
Because the silver ions are very easily reduced, it is quitedifficult to control the nucleation of Ag NPs so that it is hard toobtain the Ag NPs with narrow size distribution.35,36 In thisstudy, we prepared Ag NPs with narrow size distribution byseeding-mediated growth method.34,37 The optical propertiesof the Au-seed Ag-growth nanoparticles are similar to those ofAg-only nanoparticles because the Au seed is a very small coreat the center of the core–shell particles, and the localizedsurface plasmon resonance (LSPR) of Ag is more obvious thanAu.38 In this work, the Ag NPs of 100 nm with the absorptionpeak at ca. 510 nm were used to prepare SERS-active substratesto precisely match with the laser of 514 nm.34 TEM images andthe relevant UV-vis spectra of the Au-seed Ag-growth nano-particles are provided in Fig. 1. Fig. 1a shows the uniform AgNPs with an average diameter of 100 nm prepared by seeding-mediated growth method. The nanoparticles are quasi-spher-ical shape with a few rodlike or slice-like nanoparticles as well.The size distribution of Ag NPs characterized by dynamic lightscatting is shown in the inset of Fig. 1a, with an averagediameter of 97 nm and a Relative Standard Deviation (RSD) of19%. Fig. 1b gives the UV-vis absorption spectra of Au seedsand Ag NPs. The peak at near 520 nm is the LSPR peak of Auseeds; the peak at 510 nm with strong absorbance is contrib-uted to the Ag NPs of 100 nm. The UV-vis spectra illustratedthat the optical properties of Au-seed Ag-growth nanoparticleis dominated by Ag NPs.
3.2. Uniformity and reproducibility
As a basic requirement of SERS analysis, the uniformity andreproducibility of SERS signals play a crucial role in practicalapplication. The uniformity of SERS detection is mainlyattributed to two aspects: (1) the uniformity of the
Fig. 1 Representative TEM images of prepared Ag NPs. Inset in (a)shows the size distribution of Ag NPs. The image (b) shows the UV-visspectra of Au seeds (black) and Ag NPs (red) respectively. Photographof the two solutions (Au: brown, Ag: yellow) are shown in the inset.
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nanostructure and (2) the homogeneous distribution of probemolecules on the nanostructures, which stem from the SERSformula: ISERS ¼ EFNads(Iref/Nsol). In this work, the high pres-sure in ltration generated the closely-packed Ag NPs arrays,which ensured the uniformity of the microstructure ondifferent samples. And the uniformity can be optimized byadjusting the amount of Ag NPs. The morphologies of the ltermembrane with various volumes (ranging from 0 to 6 mL) ofAg NPs are shown in Fig. 2. It is clear that the holes made up ofnanobers are gradually lled with increasing the volume ofAg NPs. In the inset of Fig. 2, the color of the lter membranewas changed from white to silver-gray to a metallic luster dueto the increasing density of Ag NPs. When the volume of silversolution is larger than 5 mL, the holes are completely lled,and the membrane is covered with uniform Ag NPs distinctlyas shown in Fig. 2. In the process of ltration, nearly all Ag NPswere blocked by the lter membrane and formed a close-packed Ag NPs array under high pressure. The unique lter-based method provided a homogeneous and dense distribu-tion of Ag NPs which guaranteed the excellent uniformity andreproducibility of SERS signals as shown in Fig. 3. The RSD ofSERS signal gradually decreased with increasing the volume ofAg solutions and got lower than 10% at a volume larger than 5mL as shown in Fig. 3a, which matched the SEM micrographsof SERS substrate in Fig. 2. The uniform adsorption of probemolecules on Ag NPs also ensured little vibration of the SERSsignals. The adequate adsorption guaranteed the homoge-neous distribution of probe molecules on the Ag NPs, as dis-cussed in detail below. Fig. 3b is a typical Raman mappingimage with an area of 50 � 50 mm2, where the integral inten-sities of the Raman peaks of R6G at 613 cm�1 were visualizedby color. The inset in Fig. 3b showed a normal distributionhistogram of SERS intensity, and the RSD of intensity in thearea is 9% according to Gaussian tting. Raman mapping is astrict standard to characterize the homogeneity of SERSsignals over a large area, which is able to prove the uniformityof the SERS signals. Comparing different samples preparedusing the same method, the reproducibility was also studiedand the achieved RSD was 10%. The unique pretreatmentmethod in this work guaranteed the excellent performance inSERS analysis. First, the pre-adsorption of probe moleculesensures the adequate and homogeneous adsorption of probeson Ag NPs; second, the ltration can generate high pressure,which ensures the dense packing of Ag NPs and excellentreproducibility and homogeneity of the structure. Moreover,the method is extremely simple.
3.3. Optimized adsorption condition
From the view of EFs, the lter-based method generated more“hot spots” because of the three-dimensional and closely-packed Ag NPs structure which is different from previoussimilar reports.23 Just like the formula described, the ISERS isalso proportional to the amount of probe molecules underthe laser spot. So the intensity of SERS signals can bepromoted via improving the adsorption capacity of Ag NPs.The adsorption kinetics were investigated for a better
Fig. 2 Representative FE-SEM micrographs of filter membranes with different volumes of Ag NPs solution. Photographs of the membraneadsorbed Ag NPs are shown in the inset.
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understanding of the mechanism of adsorption as shown inFig. 4a. The adsorption amount of R6G was represented bythe uorescence intensity difference before (qbef) and aer(qa) adsorption. It can be seen that the adsorption amountincreased quickly in the rst 1 h and then rose slowly untilthe adsorption equilibrium was reached within 2 h. So theadsorption time was set at 2 h in order to get a uniform andadequate adsorption of probe molecules on the Ag NPs. Forthe electrostatic adsorption, the zeta potential, which canchange the adsorption capacity of the adsorbent, plays a veryimportant role in improving the adsorption performance.Fig. 4b showed the changes of the zeta potential of the Ag NPssolution and corresponding adsorption amount of R6G atdifferent pH values ranging from 3.0 to 12.0. The negativesurface charges of Ag result from the adsorbed PVP andchanges with the pH linearly until the emergence of aninection point at pH 10. The changes of adsorption amountof R6G were shown in Fig. 4b too. With the reduction of thezeta potential, the adsorption capacity to the cationic dye R6Gsignicantly improved because of the larger negative zetapotential. So the pH 10 was selected in the adsorption system.Representative SERS intensities of 10�6 M R6G usinguntreated and the optimized conditions are shown in the ESI
Fig. 3 (a) RSD of intensity variation with increasing volumes of Ag NPs soThe picture in the inset is the normal distribution curve of Raman intensitperformed.
in Fig. S1.† SERS signals were increased obviously aerpretreatment, and were almost doubled compared to theuntreated one.
3.4. Ultrahigh sensitivity
As a potential analytical tool, SERS has achieved the level ofsingle-molecule detection.39–41 The limit of detection (LOD) isan important indictor to reect the SERS sensitivity that wasestimated by the intensity of the SERS signals (ISERS). The SERSspectra at different concentrations from 1 � 10�6 to 5 � 10�14
M are shown in Fig. 5. We can see that the spectral intensitiesdecline with diluting the concentrations of R6G. Spectrum (g)in Fig. 5 gives the blank Raman spectrum without R6G. Bycomparing spectrum (f) with (g), it can be found that thetypical peaks of R6G still clearly exist at 613, 1363, 1509, 1572,and 1650 cm�1.42,43 Some spectral features of PVP adsorbed onthe Ag NPs have appeared in the spectrum (f) because of thelow concentration of R6G.26 The linear relationship betweenthe SERS intensity at 613 cm�1 and the concentration of R6G isshown in Fig. S2.† For the LOD, the intensity of spectrum (f) islarger by about an order of magnitude than spectrum (g). Sothe LOD based on this pretreatment is credible which was
lution (b) mapping image of SERS intensity of R6G (10�6 M) at 613 cm�1.y, and optical micrograph is the region where the Raman mapping was
Fig. 4 (a) Adsorption kinetics curve of R6G on Ag NPs (1.25 mg Ag NPsand 5 mL of 10�6 M R6G at 25 �C, pH 9.0) and (b) the zeta potential ofAg NPs solution and corresponding adsorption amount of R6G atdifferent pH values from 3–12 (1.25 mg Ag NPs and 5 mL of 10�6 MR6G reacted for 2 h at 25 �C).
Fig. 5 SERS spectra of R6G with different concentrations viapretreatment. Spectra (a–f) represent the concentration of R6G being10�6, 10�8, 10�10, 10�12, 10�13, 5 � 10�14 M respectively and spectrum(g) represents the blank spectrum of substrate.
Fig. 6 Fluorescence spectra of different concentrations of R6G,representing 10�6, 10�9, 10�10 M, and the 10�6 M after filtration.
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decided to be ca. 5 � 10�14 M to R6G. These results indicatethat the pretreatment method including dense SERS-activesubstrates and excellent adsorption capacity can furtherimprove the sensitivity of SERS.
3.5. Enhancement factors
In addition to the LOD, the enhancement factors (EFs) are nor-mally used to evaluate the enhanced capabilities of SERSsubstrates as well.44 The EFs were calculated by the following
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equationmentioned at the beginning: EF¼ (ISERS/Nads)/(Iref/Nsol),where ISERS is the integral intensity of 10�4 M R6G adsorbed onSERS substrate. As can be seen in Fig. S2,† the linear relationshipbetween the SERS intensity and the concentration of R6Greached an inection point at a concentration of 10�4 M. Iref isthe corresponding intensity of 10�4 M R6G in solution. Nsol andNads are the number of the probe molecules in the excitationregion of the laser beam in solution and SERS substrate,respectively. Nsol can be obtained by the following equation:Nsol¼ AhcNA, where A is the area of the focal spot of the laser, h isthe effective layer depth, c is the concentration of R6G, and NA isAvogadro’s constant.45 The quantication of Nads, the number ofprobe molecule adsorbed on the surface, is more difficult thanthe other parameters. It is difficult to determine the actual Nads
in the previous research. In this study, the Nads can be calculatedaccurately through measuring the remaining molecules in thesolution aer ltration because of our unique lter-basedpretreatment. As shown in Fig. 6, the concentration of 1 � 10�4
M R6G solution adsorbed on Ag NPs was 10�10 M aer ltration.It proved that nearly all R6Gmolecules have been adsorbed by AgNPs, so we can get the number of molecules on one nanoparticleas we known the concentration and size of Ag NPs in the solu-tion. Then the number of molecules within the area of laser spot,Nads, could be calculated precisely. So the EF of the substrate is3.22 � 108.
3.6. Stability
It is well known that silver microstructures have stronger SERSeffects than gold and other noble metal microstructures, andsilver is much cheaper than gold.34,38 However, most SERSapplications employ the Au system instead of Ag because goldnanoparticles, being mature in preparation and properties,have been in use for several decades in biology. Another reasonis the instability of silver nanoparticles.46,47 It is a problem fortheir practical application how to maintain the excellent SERSperformance of the SERS substrates made by silver for a longtime and in different storage environments. Two sets of SERS-active substrates were prepared to examine the variation of theSERS signals caused by environment and aging. Two substrates
were treated at 80 �C and 120 �C for 3 h in air atmosphere,respectively. And compared their performance with the onedried naturally at 25 �C. Aer the heat treatment, the SERSintensities are very similar between the two samples, which justdecrease a little compared with the original sample as shown inthe Fig. 7a. The RSD of SERS intensity between them is 10%,which indicates the low sensitivity of the substrates to thetemperature. Because the silver may react with oxygen or sulfurin air, the substrates made with silver will be made invalidgradually with aging in ambient atmosphere. Fig. 7b gives theSERS intensity changes of the substrates for about threemonths. Within two months, the sample shows only trivialchanges in their spectra intensity. The SERS signals, however,decreased rapidly aer 2 months, which was caused by theoxidation of the Ag NPs in the air as shown in XPS spectra(Fig. S3† in the ESI). We can conclude that the substratesprepared by this pretreatment method are hardly oxidized andare stable for about two months.
Fig. 8 (a) Effect of the electrostatics on the adsorption of melamine byAg NPs represented by the changes in UV-vis intensity. (b) SERSspectra of melamine with different concentrations via pretreatment.Spectra (a–f) represent the concentration of melamine being 10�3,10�4, 10�5, 10�6, 10�7, 10�8 M respectively, and spectrum (g) repre-sents the blank spectrum of the substrate.
3.7. Detection of melamine
The detection ability for practical applications, in particulartrace detection, is the most important criterion to evaluate theSERS performance. Melamine is a nitrogen-rich organic, andits intake has been linked to kidney stones and other healthproblems. A tolerable daily intake of 0.2 mg kg�1 body weightfor melamine was established.48 It is important to improve thetrace detection ability of melamine in SERS detection. Partic-ularly, several studies concerning melamine detection by SERSwere reported in the last few years. Several studies had indi-cated that the endocyclic N atom of melamine is easilyprotonated, which can be used to improve the adsorption ofmelamine on the Ag NPs.48,49 The UV-vis intensity difference ofmelamine before and aer adsorption is shown in Fig. 8a. At alow concentration of 10�6 M, the adsorption amount ofmelamine under acidic conditions is better than neutral andalkaline, which is due to the effect of the electrostaticenrichment. The SERS spectra at different concentrationsfrom 1 � 10�3 to 1 � 10�8 M were investigated in the spectralregion of 500–1200 cm�1, as shown in Fig. 8b. Typical Raman
Fig. 7 SERS spectra of filter-based Ag NPs substrates stored (a) under diffabout three months.
peaks of melamine at 387, 576, 686 and 983 cm�1 wereobserved in spectrum (a) and the spectral intensities declinedwith diluting the concentrations of melamine.49,50 Spectrum(g) in Fig. 8b gives the blank Raman spectrum without mela-mine. By comparing (f) with (g), it can be found that the mostintense peak at 686 cm�1 of melamine still existed clearly. Thecapacity of sensing melamine at the ppb level made the simplepretreatment method very promising for trace chemicaldetections.
erent temperatures of 25 �C, 80 �C, 120 �C and (b) for different times for
Excellent SERS performances were obtained by a simple andpractical pretreatment method based on the adjustable surfacestate and morphology of SERS-active arrays, which originatedfrom the widely known formula of EFs. This type of pretreat-ment for SERS analysis is driven by numerous advantages suchas (i) homogenous distribution of Ag NPs and adsorbed probemolecules making the detection uniform and repeatable, (ii)more “hot spots” and optimized adsorption capacity improvingthe sensitivity greatly, (iii) a unique lter-based method leadingto an accurate calculation of EFs, (iv) low sensitivity to theenvironment causing the long lifetime. The lter-based methodcombined with the optimized adsorption of the probe moleculein this work has improved the SERS performances, in particularthe sensitivity and uniformity. Furthermore, the simple fabri-cation process and high performances transform SERS into apractical detection tool that can easily be applied to the chem-ical and biological sensing.
Acknowledgements
This work was supported by the National Natural ScienticFoundation of China (21075076).
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