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Keywords:

  • surface-enhanced Raman scattering;
  • glycidyl methacrylate-ethylene dimethacrylate porous material;
  • uniform substrate;
  • inverted self-assembly method;
  • ultrasensitive detection

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental section
  5. Results and discussion
  6. Conclusion
  7. Acknowledgement
  8. References
  9. Supporting Information

Surface-enhanced Raman scattering (SERS) in practical application and theoretical research mostly depends on the performance of the SERS substrate. In this study, a new SERS substrate which is based on inverted self-assembly of Ag nanoparticles (AgNPs) on glycidyl methacrylate-ethylene dimethacrylate (GMA-EDMA) porous material is developed. The characterization results show the GMA-EDMA material with intertwined pores may contribute to the distribution of the AgNPs to fabricate an ideal substrate for SERS detection. In view of the characteristics of porous material, an inverted assembly method is proposed and used in operation to avoid the adverse gravity effect which may make the AgNPs plug up the pore channel and distribute on the surface unevenly. By the inverted self-assembly method, the AgNPs could uniformly distribute on the surface of the material stably. The prepared substrate presents ultrasensitivity and good reproducibility for SERS detection. The enhancement factor of rhodamine 6G (R6G) detection is approximately 1014 and the relative standard deviation of each characteristic peak is about 15% when the substrate is used. The substrate also shows a good performance in detecting paraquat and thymine. The ultrasensitive SERS substrate can be readily integrated into pesticide detection systems and biological sample analysis. Copyright © 2013 John Wiley & Sons, Ltd.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental section
  5. Results and discussion
  6. Conclusion
  7. Acknowledgement
  8. References
  9. Supporting Information

With the integration of high sensitivity, rapid detection and unique spectroscopic fingerprint, surface-enhanced Raman scattering (SERS) has become a powerful tool in chemical and biological analysis.[1-9] SERS enhancement mainly depends on the excitation of localized surface plasmon resonance (SPR) on the rough metallic surfaces.[10-12] It is well known that a very strong enhancement should occur in some particular sites which may be the junctions between nanoparticles (NPs).[13-15] Metal colloids such as the Ag sols are one of the most commonly used SERS substrates and salt as the aggregation agent to generate enough AgNPs junctions for SERS detection.[16, 17] In our previous study, the solution of mixing R6G, AgNPs and NaCl was spotted on glycidyl methacrylate-ethylene dimethacrylate (GMA-EDMA) porous material for ultrasensitive SERS detection.[18] However, when the Ag sols are directly used, the aggregation by NaCl solution is difficult to be controlled and it is prone to change during measurement influenced by the laser beam. Consequently, their enhancement could be changed as aggregation changes.

In order to develop a stable SERS substrate that provided strong signal enhancement, the substrates that were consisted of closely packed NPs have been studied. These substrates could provide particular enhancing at the interstitial sites between particles.[19-21] The NPs in this form should immobilize on some materials with particular structure so that the NPs could be stably connected to the material and form a good distribution. The porous materials have been applied to SERS by several groups because of their charateristic morphology and high SERS performance.[22, 23] For example, silver/silicon nanoporous pillar arrays were used for detecting rhodamine 6G (R6G) which were prepared by an immersion-plating method and the spectra with the concentration of 10−15 M could be obtained.[24] Glass nanopillar arrays with nanogap-rich Ag nanoislands could be obtained by a reactive ion etching of glass with an annealed Ag nanoisland mask and an additive Ag deposition, which was also a kind of porous structure SERS substrates. The enhancement factor of this structure was 16 times higher than that of the two-dimensional nanoislands produced by Ag film annealing.[25] The group of Kamaran J.Khajehpour prepared a gold nanothorns–macroporous silicon hybrid structure which was used as a platform for SERS detection. With this structure, 10−12 M crystal violet dye (CV) could be detected with an enhancement factor in the order of 108.[26] Lee developed a polymerization-induced phase separation method to synthesize a poly (glycidyl methacrylate-ethylene glycol dimethacrylate) porous material carrying AuNPs on the surface of the pores. The thiophenol at 10−5 M and strepatavidin functionalized with fluorescein isothiocyanate could be detected by this substrate.[27] These studies indicated that the porous materials played an important role in preparing SERS substrates. The characteristic porous structure might serve as a good media for increasing the opportunity of enhancing interparticles plasmon resonance. However, most of the substrates preparation processes were complex for practical applications; moreover, the SERS performance still need to be further improved.

An easy operation fabrication method of preparing SERS substrate with both high sensitivity and good reproducibility is significant to improve the practical application of the SERS detection. The GMA-EDMA monolithic column was introduced into analytical field in the 1990s.[28] Until now, it has been commonly used as chromatographic stationary phase which consists of highly interconnected macropores, mesopores and micropores. Such structure may generate a good distribution of AgNPs which may benefit for generating a good SERS enhancement effect. The GMA-EDMA polymer contains an active functional group of epoxy group. The amino groups can be introduced to the surface of the porous material by reacting epoxy with aqueous solution of ammonia.[29] It has a relatively strong interaction which may be electrostatic attraction between the amino group and the Au or Ag NP.[30, 31] Therefore, it is expected to prepare AgNPs modified GMA-EDMA SERS substrates by convenient and economical method of self-assembly.

In this study, we fabricated the SERS substrate by self-assembling AgNPs on GMA-EDMA porous monolithic column. An inverted assembly method was proposed and used to the assembly operation in which the AgNPs were assembled by strong interaction between the amino group and AgNP. With this method, the AgNPs were immobilized on the surface of the porous material with relatively homogeneous distribution. To investigate the performance of the SERS substrate, R6G as a probe molecule was detected. The enhancement factor was approximately 1014 for SERS detection of R6G. The signal reproducibility was pretty good for SERS detection. SERS detections of paraquat and thymine were also carried out for further assessment of the substrate.

Experimental section

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental section
  5. Results and discussion
  6. Conclusion
  7. Acknowledgement
  8. References
  9. Supporting Information

Chemicals

All chemical reagents were used as received without further purification. Glycidyl methacrylate (GMA) was purchased from Tokyo Chemical Industry Co. Ltd. (Tokyo, Japan). Silver nitrate and sodium citrate were obtained from Shanghai First Reagent Industry (Shanghai, China). Ethylene dimethacrylate (EDMA), benzoperoxide (BPO), 1-dodecanol, cyclohexanol and rhodamine 6G (R6G) were bought from Acros organics (New Jersey, USA). Ammonium hydroxide was obtained from Shanghai lingfeng Chemical Reagent Co. Ltd. (Shanghai, China). Paraquat was supplied by Agriculture Ministry of Environmental Protection and Research Institution (China). Thymine was obtained from Shanghai Jianyou Chemical Industry (Shanghai, China). The water used in all experiment was prepared with an ultrapure water purification system (SARTORIUS arium 611DI, Germany, 18.2 M•cm).

Preparation of AgNPs

The silver colloid was prepared by reaction of silver nitrate with sodium citrate on the basis of the method developed by Lee and Meisel.[32] Meanwhile, we controlled the adding time of sodium citrate to the boiling silver nitrate solution to obtain AgNPs in different size. Silver nitrate (18 mg) was dissolved in ultrapure water (50 ml) and heated to boil using a container of 250 ml round bottom flask under stirring. Under the boiling condition, solution of sodium citrate (3 ml 1.0%) was added dropwise by a syringe pump (LSPO1-1A, Baoding Longer Precision Pump Co. Ltd. China). The adding time was controlled to be 0.5 min, 5 min, 15 min, 25 min and 35 min, respectively, and then the solution was kept boiling for 60 min with continuous stirring. Characterization of the Ag colloids was analyzed by a UV–Vis spectrometer (Evolution 200, Thermo Fisher, USA), a transmission electron microscope (TEM, JEM-1400, JEOL, Japan) and a delsa nano (Beckman Coulter, USA).

Synthesis and surface functionalization of GMA-EDMA material

The GMA-EDMA porous material was synthesized as described previously.[28, 33] Here, it is described briefly. BPO was dissolved in a mixture of GMA and EDMA. The porogenic diluents consisting of cyclohexanol and 1-dodecanol were admixed slowly to the monomers. The polymerization mixture was purged with nitrogen in order to remove oxygen and enable the initiator to dissolve completely and monomers to mix homogenously. The polymerization mixture was injected into a Teflon tube which inner diameter was 1.6 mm and then sealed up. After this, the reaction was carried in a water bath spot at 72 °C for 24 h. After polymerization, in order to remove porogenic diluents, the material was washed with alcohol and ultrapure water.

The above synthetic material was a rod-shape monolithic column that contains active functional group of epoxy group. This study aimed at assembling AgNPs on GMA-EDMA porous material. The procedure of self-assembly was shown in Scheme 1. The porous material was cut into a small piece with a 4 mm height and 1.6 mm diameter. Then, one of the transverse sections of the piece was polished smooth with a 600 mesh abrasive paper; the other transverse section and side of the piece material were covered with an insulating tape (Scheme 1 (a) (b)). For surface functionalization of the porous material, the polished side of the porous material was immersed in aqueous solution of ammonia (1 M) using 0.5 ml centrifuge tube as the container and the reaction proceeded in a water bath at 60 °C for 6 h (Scheme 1 (c)). Upon completion of the reaction, the amino groups were introduced into the porous structure on surface of the material by reacting epoxy with aqueous solution of ammonia. After modified operation, the material was washed with ultrapure water to remove the excess aqueous solution of ammonia until it is neutral for further use.

image

Scheme 1. Scheme of fabrication process of the self-assembly SERS substrate.

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Inverted self-assembly of AgNPs on GMA-EDMA porous material

Into a centrifuge tube, 300 µl prepared Ag sol was added and then the amino modified porous material was inserted into it invertedly and kept in shade for 36 h. The Ag colloidal particles were adsorbed onto the material to form a self-assembly SERS substrate (Scheme 1 (d)). The particles which had not been absorbed onto the material were washed away by ultrapure water. The material following this process was prepared as a SERS substrate for detections. The morphologies of GMA-EDMA porous material and self-assembly SERS substrate were explored by scanning electron microscope (SEM, S-4800, Hitachi, Japan).

Sample preparation and SERS measurement

Solutions of R6G, paraquat and thymine were prepared at a concentration of 1 mM with ultrapure water as the solvent. Subsequently, a series of solutions at different concentrations were obtained by diluting them with ultrapure water.

Onto the self-assembly substrate surface, 70 µl solution was added. The solvent would flow away through the pores of the substrate (Scheme 1 (e)) while the analyte would stay on the surface of the substrate. SERS spectra were acquired with the use of a portable Raman instrument (i-Raman, B&W Tek Inc., USA) attached with a microscope (20× objective). Samples were detected by focusing the laser on the surface of the substrate (Scheme 1 (f)) with the integration time ranging from 5 to 15 s. The 785 nm laser line was provided by a semiconductor laser with power ca. 200 mW.

Results and discussion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental section
  5. Results and discussion
  6. Conclusion
  7. Acknowledgement
  8. References
  9. Supporting Information

Synthesis and characterization of the inverted self-assembly SERS substrate

The synthesis of SERS substrate contains three main steps that are GMA-EDMA synthesis, amino modification and AgNPs self-assembly (Scheme 1). The reaction scheme for the surface immobilization of AgNPs on the GMA-EDMA porous material interface was also shown in Fig. S1 (Supporting information).

The GMA-EDMA porous material was synthesized as described previously. Moreover, a Teflon tube was used as the reaction mould to make the porous material in a suitable size and easy to be cut without fragmenting (Scheme 1 (a)). In order to immobilize AgNPs on the GMA-EDMA porous material by the method of self-assembly, the amino group was introduced to the porous material by reacting epoxy with aqueous solution of ammonia (Scheme 1 (c)). Elemental analysis result of the amino modified material showed the nitrogen content was 0.98% after washing out of the residual aqueous solution of ammonia with ultrapure water. This data indicated that the amino group has been successfully introduced onto the surface of the GMA-EDMA porous material. The modified porous material was immersed into the Ag colloid for further self-assembly operation. In this self-assembly step, it should be noticed that the GMA-EDMA material has an obvious characteristic of porous. Therefore, if we use traditional method of facing up self-assembly,[34, 35] the effect of gravity might make the AgNPs plug up the pore channel and distribute on the surface unevenly. Moreover, some NPs should be deposited on the porous material and led to poor reproducibility of SERS signals.

The reason of AgNPs deposition is the gravity adverse effect that makes NPs distribute on the surface unevenly. In order to solve this problem, inverted self-assembly method was applied to avoid the gravity adverse effect. The rod of the modified material was immersed into the Ag gel solution upside down that could be seen in the Scheme 1 (d). AgNPs might be absorbed to the surface of the material by diffusive force and electrostatic attraction. After the inverted self-assembly operation, unabsorbed particles were washed away by ultrapure water. There was no change of AgNPs which were self-assembled on the material in macroscopic view after washing; such phenomenon indicated that the AgNPs immobilized on the material steadily.

The GMA-EDMA material and materials with facing up and inverted self-assembly were characterized by SEM. The SEM images were shown in Fig. 1. The GMA-EDMA material showed an obvious characteristic of porous (Fig. 1(a)). Both of facing up and inverted self-assembly methods can immobile AgNPs on the surface of the material clearly (Fig. 1(b) and Fig. 1 (c)); however, the two images had an obvious difference. The AgNPs deposition took place and led to uneven distribution by facing up self-assembly (Fig. 1(b)), while with the method of inverted self-assembly, the AgNPs were evenly distributed on the surface of the GMA-EDMA material (Fig. 1(c)). The above mentioned indicated that the method using amino modification and inverted self-assembly could make the AgNPs immobilize on the GMA-EDMA porous material steadily and uniformly.

image

Figure 1. SEM image: (a) GMA-EDMA monolithic column material, ×1K; (b) SERS substrate prepared by the method of facing up self-assembly, ×20K; (c) SERS substrate prepared by the method of inverted self-assembly, ×20K; (d) SERS substrate prepared by the method of inverted self-assembly, ×5K. The inset is partially enlarged SEM image, ×50K.

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SERS activity of the R6G detected on the inverted self-assembly substrate

To compare with the performance of this prepared substrate, a traditional SERS detection of R6G solution was carried out. For the detection, a mixture of R6G, Ag colloid and sodium chloride was prepared, where the sodium chloride was the aggregating agent. Several drops of the solution were spotted onto a quartz plate; the SERS spectrum of R6G on the quartz plate was obtained as shown in Fig. 2(a). From the spectrum of 10−7 M R6G solution, signals of R6G could be identified clearly, whereas the enhanced signals could almost not be observed when the R6G concentration reduced to 10−10 M.

image

Figure 2. Raman spectra of R6G detected on the substrate: (a) 10−7 M R6G detected with traditional method of using Ag colloid as the substrate; (b) 70 µl ultrapure water detected on the inverted self-assembly substrate; (c)–(e): R6G at different concentrations detected on the inverted assembly substrate.

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In this paper, R6G was also used as the probe molecules to study the SERS properties of the inverted self-assembly substrate. With the substrate, SERS spectra of R6G with various concentrations from 10−10 M to 10−17 M were measured and presented in Fig. 2. As shown in Fig. 2, the characteristic peaks of R6G were the same to the bands of 10−7 M R6G using the traditional detection method. The Raman bands at 567 cm−1, 609 cm−1, 1307 cm−1, 1361 cm−1 and 1508 cm−1 were attributed to R6G molecule and agree with the results reported in literature.[36-40] In order to check a possible pollution and disturbance of the substrate in R6G detection, a blank sample of ultrapure water was prepared and detected with the self-assembly substrate. As could be seen from Fig. 2 (b), the bands were mainly due to the substrate; there was no signal interference of the substrate to the detection of the probe molecule. It is very interesting to observe that as low as 10−17 M R6G solution yielded clear enhanced signals. There is no doubt that the substrate prepared with the proposed method was efficient for ultrasensitive SERS detection. It is believed that the good performance of the ultra sensitivity should be attributed to the characterization of GMA-EDMA porous material. The porous structure of the material would make a certain enrichment of the sample. However, the contribution of the enrichment to the sample signal enhancement was limited because the sample dosage used was only 70 µl. The pores had uneven surface, irregular shapes and different intertwined sizes so that AgNPs immobilized on the pores had more orientation to couple with others on the three-dimensional space to generate a good SERS performance.

The SERS enhancement factor was calculated by the equation: EF = (ISERS/ NSurf) / (IRaman/NVol),[41] where ISERS and IRaman are the intensities of the same characteristic band in the spectra of the probe molecule detected by SERS and normal Raman, respectively. NSurf and NVol are the number of the probe molecules in the laser spot. In this study, the characteristic band at 609 cm−1 in the SERS spectra of 10−17 M R6G absorbed on the SERS substrate and the normal Raman spectra of 10−2 M R6G solution deposited on the quartz plate was used to calculate the enhancement factor of R6G detected on this prepared SERS substrate. The calculation is presented as following: (ISERS/ NSurf) = 8373.07 / (70 µl × 10−17 M × 6.02 × 1023 × π × (75 µm/800 µm)2); in this, we estimate the laser spot radius is 75 um, and the substrate radius is 0.8 mm. The number of the probe molecules in the laser spot is calculated as the average number of molecules; (IRaman/NVol) =10−2 M × 6.02 × 1023 × π × (75 µm)2 × 150 µm; in this, we calculate the probe volume by assuming that it is a cylinder with a radius of 75 µm and a height of 150 µm. After calculation, the SERS enhancement was estimated to be 1.1 × 1014. This value indicated that the prepared substrate has high performance in SERS detection.

SERS performance of substrates fabricated with different sizes of AgNPs

To investigate how the different sizes of AgNPs affect the SERS performance, AgNPs with different sizes were used to the inverted self-assembly operation. First, the Ag colloid was prepared by reaction of silver nitrate with sodium citrate. Meanwhile, we controlled the adding time (0.5 min, 5 min, 15 min, 25 min, 35 min) of sodium citrate to the boiling silver nitrate solution to obtain AgNPs in different size. The UV–Vis absorption spectra of Ag colloids (diluted to 10% with ultrapure water) prepared with different sodium citrate adding time were presented in Fig. 3 (a). The wavelength of the maximum absorption provides information on the average particle size and the full width at half-maximum can be used to estimate the distribution of particles.[42] It could be seen from the spectra that with the sodium citrate adding time increased, the wavelength of maximum absorption took bathochromic shift gradually which was related to the average particle size increased, whereas the full width of absorbing increased showed that the distribution of particles broadened correspondingly. This result could partly be explained. When the adding time of the sodium citrate was short, the concentration of the reducing agent increased rapidly. Therefore, the rate of generating the AgNP crystal nucleus increased correspondingly which was much higher than that of the grain growth of the AgNPs. Consequently, a large number of small AgNPs with high specific surface area could generate in a short time. With the sodium citrate adding time increasing, the AgNP crystal nucleus at the initial phase of the reaction reduced accordingly. Along with the progress of the reaction, NPs continued to grow in the original crystal nucleus which resulted in the generation of larger particles with wider distribution. A TEM image of the Ag colloid which used 0.5 min sodium citrate adding time was presented to study the morphologies of the NPs. As could be seen from Fig. 3 (b), the particles have a relatively good distribution of 50 nm average particle size. The particle size distribution of the others was measured with the delsa nano as shown in Fig. S2 (Supporting information). It shows that the sizes of Ag colloid synthesized with different sodium citrate adding time of 0.5 min, 5 min, 15 min, 25 min and 35 min are 51.6 nm, 53.8 nm, 60.5 nm, 75.4 nm and 87.8 nm, respectively.

image

Figure 3. Characterization of the Ag colloids: (a): UV–Vis spectra of the Ag colloid synthesized with different sodium citrate adding time: from a to e: 0.5 min, 5 min, 15 min, 25 min, 35 min; (b): TEM image of Ag colloid synthesized with 0.5 min sodium citrate adding time. This figure is available in colour online at wileyonlinelibrary.com/journal/jrs

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Finally, it could be seen from Fig. 3 that with the sodium citrate adding time increased, the average particle size increased gradually. In order to obtain substrates containing AgNPs of different sizes, the prepared AgNPs with different sizes were used to the inverted self-assembly operation. R6G solution (10−17 M) was used to study the SERS performance of the substrates. The Raman spectra were presented in Fig. S3 (Supporting information). Each spectrum was the average spectrum of the solution which was measured at five different positions on the substrate surface. As could be seen from the spectra, in this small particle size range, the bigger size led to stronger SERS signal which might attribute to the change in the frequency and the strength of the resonance of the substrate,[43] but the SERS performance of the substrate seemed to be changed not much (whole substrates could obtained the signal of 10−17 M R6G). The signals of the impurities such as the citrate absorbed on the AgNPs were strong when the AgNPs with large size were used. In addition, with the size increased, the reproducibility of sample signal which was measured at different positions on the substrate became poor correspondingly. The reason was that during synthesis of Ag colloid, the distribution of particles size became wide along with the particles size increase, which led to the decreasing uniformity of the substrate.

In this study, we wanted to synthesize a SERS substrate which had ultrasensitive performance in sample detection with good signal reproducibility. Consequently, 0.5 min adding time of sodium citrate was selected to prepare AgNPs with the size of 50 nm sizes and a narrow size distribution. The different amplification SEM images of the prepared substrate were presented in Fig. 1 (d). It indicated that on a large area of the material, the AgNPs were immobilized on the GMA-EDMA porous material uniformly. According to the scale of the partially enlarged SEM image (Fig. 1 (d)), the sizes of the AgNPs immobilized on the material were 30 to 80 nm which were similar to that of the prepared AgNPs in colloid state (Fig. 3 (b)). To investigate the reproducibility, SERS spectra of 10−17 M R6G were measured at eight different positions on the substrate. The spectra were presented in Fig. S4 (Supporting information) that looked very uniform. Calculating relative standard deviation (RSD) of each characteristic peak, the RSDs were from 14.19% to 23.22% and most were around 15%. These data indicated that the substrate had a good reproducibility for SERS detection. Comparison of the SERS performance of the substrates fabricated by inverted self-assembly and facing up self-assembly. SERS spectra of 10−17 M R6G could also be obtained by the latter. However, the reproducibility of the signal became poor with the RSD about 36% when the analyte was detected on the substrate which fabricated by the facing up self-assembly method.

SERS activities of paraquat and thymine with the inverted self-assembly substrate

The SERS measurement of R6G is a good evidence to indicate the efficient SERS performance in ultra sensitivity and good reproducibility using the substrate. In order to further investigate the performance of the substrate, we applied the substrate to the SERS detections of paraquat and thymine.

Paraquat is a kind of steriant herbicide that exhibits a high toxicity. More than 20 countries have restricted or prohibited the use of it. We first used a normal SERS method to detect paraquat using Ag colloid as a substrate and adding sodium chloride for AgNPs aggregating, and found that as low as 10−5 M paraquat on the quartz plate could be detected with the normal SERS method. Then, the inverted self-assembly SERS substrate was applied to the paraquat detection. The measured SERS spectra were presented in Fig. 4. It could be seen that the signal of 10−8 M paraquat was clear using the substrate that was clearly more sensitive than the traditional Ag colloid. The RSD values of the characteristic peak intensity at 837, 1187, 1292 and 1645 cm−1 were 12.76%, 15.62%, 10.15% and 16.68%, respectively. The reproducibility was still very good.

image

Figure 4. Raman spectra of paraquat detected on the substrate: (a) 10−5 M paraquat detected with traditional method of using Ag colloid as the substrate; (b) 70 µl ultrapure water detected on the inverted self-assembly substrate; (c), (d): Paraquat at different concentrations detected on the inverted assembly substrate.

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Thymine is one of the four nucleosides which are the basic components of DNA and plays important roles in genetic expressions and replications. The concentration of thymine which could be detected on Ag colloid SERS substrate has been reported to be 10−4 M,[44] in which the sodium chloride was used for AgNPs aggregating. This study used the inverted assembly SERS substrate for further detection. As could be seen in Fig. 5, the signal of 10−4 M thymine was clear using this substrate. When thymine at low concentration of 10−6 M was detected by using this substrate, the characteristic peaks of the thymine could be identified clearly, in which the other bands were assigned to the signal of the GMA-EDMA materials and some impurities such as the citrate absorbed on the AgNPs. This substrate presented a better SERS performance than that of the traditional Ag colloid. We measured eight SERS spectra of 10−6 M thymine at different positions on the substrate to investigate the reproducibility. The RSDs of the characteristic peak intensities at 781, 1218, 1345 and 1650 cm−1 were 12.39%, 16.36%, 13.25% and 12.78%, respectively, which was pretty good for SERS detection.

image

Figure 5. Raman spectra of thymine detected on the substrate: (a) 10−3 M thymine detected with traditional method of using Ag colloid as the substrate; (b) 70 µl ultrapure water detected on the inverted self-assembly substrate; (c): 10−4 M thymine detected on the inverted assembly substrate. (d): 10−6 M thymine detected on the inverted assembly substrate.

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Conclusion

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental section
  5. Results and discussion
  6. Conclusion
  7. Acknowledgement
  8. References
  9. Supporting Information

We have developed an immobilized SERS substrate which has ultrasensitive performance in sample detection with good signal reproducibility. To prepare the SERS substrate, the inverted assembly method was proposed and used in operation in view of the characteristics of porous material to avoid the adverse gravity effect. In addition, the amino groups were modified to make AgNPs immobilized on the GMA-EDMA porous material surface. Following this method, the AgNPs were evenly distributed on the surface of the material. With this substrate, the SERS signal of 10−17 M R6G was observed and the enhancement factor was 1014. The RSD of each characteristic peak was about 15%. The substrate also showed a good sensitivity and reproducibility in detecting paraquat and thymine. The method of inverted self-assembling AgNPs on the GMA-EDMA porous material provided a simple and practicable way to fabricate a substrate for high performance SERS detection.

Acknowledgement

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental section
  5. Results and discussion
  6. Conclusion
  7. Acknowledgement
  8. References
  9. Supporting Information

This work was supported by the National Natural Science Foundation of China (20975039).

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental section
  5. Results and discussion
  6. Conclusion
  7. Acknowledgement
  8. References
  9. Supporting Information

Supporting Information

  1. Top of page
  2. Abstract
  3. Introduction
  4. Experimental section
  5. Results and discussion
  6. Conclusion
  7. Acknowledgement
  8. References
  9. Supporting Information
FilenameFormatSizeDescription
jrs4418-sup-0001-figures.docxWord document1809KSupporting info item

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